Add the ability to update type public status

Add the ability to update type public status

Currently, the v2 volume type update api doesn't support updating
volume type's public status. All volume types created are public
by default if not specified. It is not possible to update an existing
volume_type is_public status. It is necessary to add updating public
status for volume type. If a volume type updated from public to
private, the volumes previously created with this type will not be
affected, but the user without access will not be able to create
volume with this type anymore.

APIImpact: Add 'is_public' for v2 type update api.
Implements blueprint: volume-types-public-update

Change-Id: I4a5b2cf8f747c87908a09835a5edb8873c4e946b