Fix the way of deleting metadata properties

Since glanceclent provides special parameter for removing
metadata, method that does it in murano was updated.
Previos version was not working - the operation finished
without errors, but metadata still presented.

Closes-Bug: #1342749

Change-Id: I493a7a28c6886c948b1c71e10b652d9b7dcdffcb
Merge "Fix the way of deleting metadata properties"