Scenario manager: don't access glance_client if Glance is not available

Without this patch, if Glance is not enabled, then no Tempest scenario
can be executed because scenario/manager.py tries to access the
`image_client` attribute during its initialization. But `image_client`
is defined only if CONF.service_available.glance is true.

Change-Id: Ib8d317608ac6a13438581f389b42064772908ef9
Closes-Bug: #1513474
1 file changed