Adds new API for share metadata operating

Adds command 'metadata' for setting and unsetting metadata keys.
Adds command 'metadata-update-all' for updating all metadata.
Adds command 'metadata-show' for viewing share metadata.
Adds key '--metadata' to command 'create'

Partially implements bp manila-client-enhancements
Change-Id: I10d20e12e71065d6e40be062715fa01998828ab2