Support message queue clusters in inter-cell communication

Since cells use oslo.messaging to specify and store the message queue URL,
multiple hosts can be specified by manually modifying that URL in the database.
However, there is no way to specify multiple hosts during cell creation phase.
This patch adds a --broker_hosts option to `nova-manage cell create` command,
which is analogous to the rabbit_hosts option in nova.conf and can be used to
specify multiple message queue servers as a comma separated list. Each server
is specified using hostname:port with both being mandatory. The existing
--hostname and --port options continue to remain but are only considered if no
--broker_hosts is specified.
Internally, each host is converted to a oslo.messaging.TransportHost
and added to the generated TransportURL.
This patch also adds unit tests for creation of the TransportHosts from
user given input.

Change-Id: I14de860b1d12f3e2c0169b58651d580792d6ce0e
Closes-Bug: 1178541
Merge "Support message queue clusters in inter-cell communication"