Add flag to set nova v3 api for tempest

This commit adds a new env variable, TEMPEST_NOVA_API_V3 to set
whether the tempest api v3 tests are enabled.

Change-Id: If14deb80f271061e70fae6c3d13b210ac0dd9399
Invoke create_nova_conf_neutron from odl-compute post-install
only if nova is enabled.

Change-Id: I4627ab97eccd7fd4a2ffc31b6a0bb939b2343659
Add optional repo enablement for RHEL 7

The RHEL 7 and RHEL 6 optional repos have different names. This enables
the correct repo depending on the release.

Change-Id: I6772c9903a5b9e98cb870d586629564126a6119a
Merge "Add flag to set nova v3 api for tempest"
Merge "Invoke create_nova_conf_neutron from odl-compute post-install only if nova is enabled."
Merge "Add optional repo enablement for RHEL 7"