Add MAAS support

At the moment, Watcher can use a single bare metal provisioning
service: Openstack Ironic.

We're now adding support for Canonical's MAAS service [1], which
is commonly used along with Juju [2] to deploy Openstack.

In order to do so, we're building a metal client abstraction, with
concrete implementations for Ironic and MAAS. We'll pick the MAAS
client if the MAAS url is provided, otherwise defaulting to Ironic.

For now, we aren't updating the baremetal model collector since it
doesn't seem to be used by any of the existing Watcher strategy
implementations.

[1] https://maas.io/docs
[2] https://juju.is/docs

Implements: blueprint maas-support

Change-Id: I6861995598f6c542fa9c006131f10203f358e0a6

tox.ini

@@ -14,6 +14,7 @@ setenv =
 deps =
     -r{toxinidir}/test-requirements.txt
     -r{toxinidir}/requirements.txt
+    python-libmaas>=0.6.8
 commands =
   rm -f .testrepository/times.dbm
   find . -type f -name "*.py[c|o]" -delete