GET servers API sorting REST API updates

Updates the v2 and v3 /servers and /servers/detail APIs to support the
multiple sort keys and sort directions (using the 'sort_key' and
'sort_dir' parameters); these parameters can be specified multiple
times to create a list of sort keys and directions. These parameters
are passed from the API layer to the compute layer, then to the
instance layer (with updated version), then to the database layers,
and then to the common paginate_query function; the paginate_query
function already supports multiple sort keys and directions. The
function signatures in these various layers are updated with new
'sort_keys' and 'sort_dirs' parameters that represent the sort keys
and directions information as lists.

This support is enabled on the v2 API by the existence of a new
'os-server-sort-keys' API extension and is always enabled in the v3
API. The extension API sample issues 2 server creates and then ensures
that both servers are listed (name is unique) in the list reply.

DocImpact: The existing v2 and v3 servers API documentation needs to
reflect these new parameters.

The nova client will also be updated to use these parameters.

Change-Id: I02baf6c3cc7d29abab132ef1726140c57e17d9b6
Partially implements: blueprint nova-pagination
Merge "GET servers API sorting REST API updates"