Forbid replica provisioning from replica site

Reasons:
- Replication V1 should allow only flat hierarchy - one root replica
  with multiple read replicas. Trove should forbid attempts to create
  slave of slave of slave and so on.

Changes:
 - adding validation step that forbids read replica provisioning if passed
   replica source (slave_if_id) is not an actual source (is a read replica any
   other replica source)
 - unit test added

Co-Authored-By: Iccha Sethi <iccha.sethi@rackspace.com>

Change-Id: Ic467b2eb01194f95a30f7ce9d921d3fa942b086e
Closes-Bug: #1357707