This change creates a pre-delete hook to clean out all entries
in the credential table of the keystone database when the
keystone service is deleted. Note that these are not
the typical username/password.[0]
This fixes the issue of leftover credential blobs being saved
in the database that are unable to be decrypted since the
original encryption keys are removed upon deletion of the
keystone service
[0] https://specs.openstack.org/openstack/keystone-specs/specs/keystone/newton/credential-encryption.html
Change-Id: I8adf0878af2f3b880e9194a6cb8d97b58d6895a5
3d6f3088a3