Allow devstack to install tacker, keystone and horizon only

[yong sheng gong]

Problem:
 tacker itself does not need any other components than keystone, horizon, and tacker server. The devstack plugin installs all components, cannot install a tacker only env.

expectation:
 install a tacker only env, which just have keystone, horizon and tacker

solution:
 add a tacker_mode = isolated to devstack plugin, tacker_mode is defaulted to 'all' which complies to the services defined in local.conf. If tacker_mode = isolated, the devstack plugin will disable all other services and just enable related services

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/386388

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/386388
Committed: https://git.openstack.org/cgit/openstack/tacker/commit/?id=1de9a991df093b8b3a31dbd7ed75f17a0bb02b1d
Submitter: Jenkins
Branch: master

commit 1de9a991df093b8b3a31dbd7ed75f17a0bb02b1d
Author: gong yong sheng <[e-mail]>
Date: Fri Oct 14 05:24:58 2016 +0000

    Allow to install tacker only env

    Introduce a TACKER_MODE variable. If this is set to 'all', the
    tacker devstack plugin will install other components just like
    previous way. If it is set to 'standalone', the tacker devstack
    plugin will disable all other services and enable the mysql,
    keystone, tacker and horizon services.

    Change-Id: I5d7613566ed45aa273848bfe85d76c7a7fe97ba1
    Closes-bug: 1633327
    Closes-bug: 1620163

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/tacker 0.6.0 release.