nova-compute failed to start in OFFLINE mode

[SamP]

nova-compute failed due to following error.
ERROR nova.virt.driver ImportError: No module named libvirt

Change introduced in [1] uninstall libvirt-python and reinstall it.
However, OFFLINE mode skip the pip install.
We should skip the pip uninstall in OFFLINE mode.

[1] https://review.openstack.org/#/c/463216/

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/490364

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/490364
Committed: https://git.openstack.org/cgit/openstack-dev/devstack/commit/?id=87d2396d225533aefd8d52bb5fa574ea57dbb987
Submitter: Jenkins
Branch: master

commit 87d2396d225533aefd8d52bb5fa574ea57dbb987
Author: Sampath Priyankara <email address hidden>
Date: Thu Aug 3 16:12:40 2017 +0900

    Don't uninstall pip packages if OFFLINE=True

    lib/nova does a pip re-install of libvirt-python to rebuild the python
    library incase the underlying libvirt version changed during package
    installs. In offline mode, the underlying version of libvirt can't
    have changed; so we have the situation that we've removed the libvirt
    python bindings but can't reinstall them (because we're offline).

    This fixes that particular situation, but skipping uninstalls in
    offline mode seems generically OK.

    Change-Id: I2b75d45d94d82f87d996c7570c125d46f5f99f6a
    Closes-Bug: #1708369