Pass volume_api to get_encryption_metadata

The module encryptors was creating a volume API instance in the module
scope which caused all the modules importing it to depend on
cinderclient.

This was affecting scheduler and cert services which at some point import
the compute manager module only to access their config options. It makes
no sense to force scheduler and cert services to require cinderclient.

This patch makes the callers of get_encryption_metadata to pass the
volume api object to this method to prevent this dependency.

Closes-Bug: #1246103
Change-Id: I9eb4ae3754fa2a5ac646560a62477d6ed672e272
(cherry picked from commit a3aeace9afd5c533f040452f33482fdb55d93927)
Merge "Pass volume_api to get_encryption_metadata" into stable/havana