Honor metadata_encryption_key in glance domain

Since v2 is not using the registry client, it was not properly
encrypting image locations before storing them in the database. With
this change, the db layer in the glance domain now uses the metadata
encryption key as well.

To make this work, the declaration of the metadata_encryption_key option
had to move to glance.common.config to avoid circular module
dependencies.

Fixes bug 1128650

Change-Id: I3bb5da92ffda7bfe1bc064d11da8ecd4e6d9ab1d
Merge "Honor metadata_encryption_key in glance domain"