The option 'allow_inuse_volume_type_modification' is
introduced as a bridge to not break upgrades without
notice. It is introduced in Ocata release [1], so it
is time to remove this option.
[1] https://review.openstack.org/#/c/440680/
Change-Id: I8c61b11994c1f8f571e65cefe4965f7d69b13100