This adds support, in a new microversion, for specifying an availability
zone to the unshelve server action when the server is shelved offloaded.
Note that the functional test changes are due to those tests using the
"latest" microversion where an empty dict is not allowed for unshelve
with 2.77 so the value is changed from an empty dict to None.
Implements: blueprint support-specifying-az-when-restore-shelved-server
Closes-Bug: #1723880
Change-Id: I4b13483eef42bed91d69eabf1f30762d6866f957