Improve pip retrieval macro

[Gaël THEROND]

What happened:

When performing an offline build of the kolla containers (mirroring everything internally PYPI/REPOS), the
get_pip() function from the macros.j2 template can't reach the internet and so can't install python-pip and fail.

What should happen:

Because major distribution provides a python-pip package, even if there is situations where this is not possible, we should retrieve pip from those distribution repositories using the available package manager and then fallback to this external retrieval if not applicable.

That would allow offline build based on package manager to easily and seamlessly retrieve pip without having to use an external URL or a HTTP(s) proxy.

Some offline installation don't allow any kind of HTTP proxy but yet provide either internal mirrors of a distribution repositories and/or a PYPI mirror/proxy that specifically can't be used as a HTTP(s) proxy (Nexus python proxy for instance).

How to reproduce:

Launch a build of the kolla containers in an offline mode, pip won't install as containers are not able to reach internet and consequently fail the build.

**Environment**
OS: CentOS-8.1.1911
Kernel:4.18.0-147
Docker version: 19.03
Kolla version: 9.2.0

[Gaël THEROND (gtherond)]

Gonna work on this patch.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/751615

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/751706

[OpenStack Infra (hudson-openstack)]

Change abandoned by Gaël THEROND (<email address hidden>) on branch: master
Review: https://review.opendev.org/751706
Reason: conflict with already existing change: https://review.opendev.org/#/c/751615/

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/751742

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/751765

[OpenStack Infra (hudson-openstack)]

Change abandoned by Gaël THEROND (<email address hidden>) on branch: master
Review: https://review.opendev.org/751765
Reason: Will push again with a better commit tree.

[OpenStack Infra (hudson-openstack)]

Change abandoned by Gaël THEROND (<email address hidden>) on branch: master
Review: https://review.opendev.org/751742
Reason: Will push again with a better commit tree.

[OpenStack Infra (hudson-openstack)]

Change abandoned by Gaël THEROND (<email address hidden>) on branch: master
Review: https://review.opendev.org/751615
Reason: Will push again with a better commit tree

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/751787

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/752455

[OpenStack Infra (hudson-openstack)]

Change abandoned by Gaël THEROND (<email address hidden>) on branch: master
Review: https://review.opendev.org/752455
Reason: improper commit message.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/751787
Committed: https://git.openstack.org/cgit/openstack/kolla/commit/?id=b93c40a3635fbce72e48dc29120e50484cba36ca
Submitter: Zuul
Branch: master

commit b93c40a3635fbce72e48dc29120e50484cba36ca
Author: Gaël THEROND (Fl1nt) <email address hidden>
Date: Thu Sep 17 14:38:21 2020 +0200

    Improve pip install process for offline deployment.

        * "Use distribution-provided pip"
        * "Use python's pip module invocation method"
        * "Install pip earlier in order to avoid multiple installation"
        * "Remove pip_version variable requirement and call"

    Change-Id: Id0e738044a1931f9d611a7281a48ea4a593f1cf1
    Closes-bug: #1893204

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/ussuri
Review: https://review.opendev.org/758054

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/train
Review: https://review.opendev.org/758057

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/758054
Committed: https://git.openstack.org/cgit/openstack/kolla/commit/?id=943c03f2ea455dfa583f065bdc42168c8def448e
Submitter: Zuul
Branch: stable/ussuri

commit 943c03f2ea455dfa583f065bdc42168c8def448e
Author: Gaël THEROND (Fl1nt) <email address hidden>
Date: Thu Sep 17 14:38:21 2020 +0200

    Improve pip install process for offline deployment.

        * "Use distribution-provided pip"
        * "Use python's pip module invocation method"
        * "Install pip earlier in order to avoid multiple installation"
        * "Remove pip_version variable requirement and call"

    Change-Id: Id0e738044a1931f9d611a7281a48ea4a593f1cf1
    Closes-bug: #1893204
    (cherry picked from commit b93c40a3635fbce72e48dc29120e50484cba36ca)

[OpenStack Infra (hudson-openstack)]

Change abandoned by Mark Goddard (<email address hidden>) on branch: stable/train
Review: https://review.opendev.org/758057

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/kolla 10.2.0 release.

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/kolla 9.3.1 release.