Freeze functionality in the replication feature doesn't work as
expected, since it is not being used on the scheduler to exclude
backends or used on the API or volume nodes so API-to-Vol operations
like delete and create snapshot will also work.
This patch fixes the freeze mechanism by excluding frozen backends in
the scheduler and checking the if the service is frozen on all other
modifying operations.
Since extend operation now goes through the scheduler it will be frozen
there.
Closes-Bug: #1616974
Change-Id: I4561500746c95b96136878ddfde8ca88e96b28c6
2195885e77