Remove XML support from some v3 API plugins

Remove XML support from the following v3 compute API plugins:

 - access_ips
 - agents
 - availability_zone
 - block_device_mapping
 - cells
 - extended_availability_zone

Part of blueprint remove-v3-xml-api

DocImpact

Change-Id: I9cf3017668774e93d4679067e41da2c8301606a0
Merge "Remove XML support from some v3 API plugins"