Allow deployers to choose which clients they want to support

At the moment, when an OpenStack deployer installs heat, it has all of the OpenStack python clients as dependencies. Some deployers may choose run to run a slimmer deployment (nova/keystone/cinder/swift/glance) and they may not need all of those clients.

Some of the clients, like python-monascaclient, pull in dependencies like cassandra-driver, which take 4-6 minutes to build in the OpenStack gates.

Would it be possible to use an [extras] section similar to what gnocchi is doing? https://github.com/gnocchixyz/gnocchi/blob/master/setup.cfg#L21-L73

It would allow a deployer to do something like:

heat[nova,glance,keystone,cinder,swift]