Allow unshelve to a specific host (REST API part)

This adds support to the REST API, in a new microversion, for specifying
a destination host to unshelve server action when the server
is shelved offloaded.
This patch also supports the ability to unpin the availability_zone of an
instance that is bound to it.

Note that the functional test changes are due to those tests using the
"latest" microversion 2.91.

Implements: blueprint unshelve-to-host
Change-Id: I9e95428c208582741e6cd99bd3260d6742fcc6b7

doc/api_samples/versions/versions-get-resp.json

@@ -22,7 +22,7 @@
                 }
             ],
             "status": "CURRENT",
-            "version": "2.90",
+            "version": "2.91",
             "min_version": "2.1",
             "updated": "2013-07-23T11:33:21Z"
         }