[manila][ci] Replace legacy jobs
[manila][ci] Replace legacy jobs

- manila-tempest-minimal-dsvm-cephfs-native and
  manila-tempest-minimal-dsvm-cephfs-nfs are both
  legacy jobs that use the now deprecated
  devstack-gate.
- We no longer need the CentOS based experimental
  jobs for manila since the manila team plans to
  delete them from their tree. Newer CentOS experiments
  can happen with the native zuulv3 style jobs.
- The jobs added are inheriting manila-tempest-plugin jobs,
  but currently don't override anything - there's a plan
  to supply overrides soon to accommodate the ceph support
  matrix that the manila team wants to test via this
  repository.
- Also make minor refactors to existing base
  jobs to make reuse of certain things easier

A future commit will remove the pre-test-hook and
post-test-hook scripts; they're presently there to allow
testing legacy jobs against older releases of
openstack/manila.

Change-Id: I20ab47cf7846e7c0291ef4771cc93fd85d0452b0
Signed-off-by: Goutham Pacha Ravi <23768c94a9ff9c3948f2b1373d75b7bec9121063@gmail.com>

openstack/devstack-plugin-ceph