Add extra-specs support for volume-type

changed to volume-type
changed command to --long, added a formatter for key=value
Just noticed there is an option in cinder for a command called
extra-specs-list, only relates to volume-types, to list the
meta-data key/value pair. Added that functionality.
Also made minor changes so delete/set/unset can be called by name.

Change-Id: If534ccd1d8a3abc6d235c60ec9a964b88e3fa66d