Correct parsing of volume metadata

Volume metadata supplied via the command line is incorrectly parsed,
converting each character into a key with “None” as the corresponding
value.  This bug fix corrects the parsing and also includes regression
tests when metadata is provided when creating a volume.

Change-Id: I8c093df59ea4de2e2a1b7cf3ac7c4624c159fb66
Implements: blueprint encrypt-cinder-volumes