This patch modifies the delete volume type logic such that a volume type
cannot be deleted if:
1) It is the default volume type
2) The default type configured is wrong/doesn't exist
This also implies that there will exist atleast 1 volume type in the
deployment and that will be the default volume type.
This also includes following 2 changes on the default_volume_type conf option:
1) It is a mandatory field
2) default value of this config option is '__DEFAULT__'
All these changes ensure that we don't allow creating untyped volumes.
Also it is now possible to delete the '__DEFAULT__' type as it acts
as a normal type.
Change-Id: Ifa3d22305060b5913332cad89ea696bf7fd84ce1
Closes-Bug: #1886632
e5d842eb1b