Fix test variable injection in CI
Fix test variable injection in CI

* Migration is not a multi-backend only feature.
* Snapshot tests, Shrink tests and Quota test flags
  should have appropriate devstack variables.

Closes-Bug: 1652150

Change-Id: I7a83981045c03bcc605ec0119a4b425fb5f9e987

vponomaryov/manila,openstack/manila,vponomaryov/manila,openstack/manila,bswartz/manila,bswartz/manila