Add test case for volume_encryption_metadata_get

Add test case for volume_encryption_metadata_get

This change adds unit tests for the volume_encryption_metadata_get
function. The unit tests provide protection against regressions when
refactoring this code as part of follow-up patches.

Writing the unit tests also exposed a minor issue with the existing
implementation of the volume_encryption_metadata_get function. If the
volume type is not encrypted, then the existing implementation would
raise an exception due to volume_type_encryption_get returning None.
In practice, this issue would not be encountered due to separate
checks to ensure that the volume type is encrypted, but a small
refactoring obviates the need for these checks and allows the
volume_encryption_metadata_get function to be invoked for both
encrypted and "normal" volumes.

A separate patch will clean up the unnecessary checks to ensure that
the volume type was encrypted prior to calling this function.

Related to blueprint volume-encryption-metadata

Change-Id: I6ba62a58ac62a3b7bfb2b9d4d18b37ef458b616c