This change allow API user to remove the consistency group name
or description information.
If user use the `cinder consisgroup-update consisgroup-id --name ''`
or `cinder consisgroup-update consisgroup-id --description ''`
to update the consisgroup's name or description information,
the consisgroup's name or description information will be removed.
APIImpact
Change-Id: I0d661775994d6de580f8788397f0e91e90148edd
Closes-Bug: #1572986