Add support for availability zone

This is needed to choose which compute node you will boot on.
Not added test to create_server, this is tech debt
we should address soon.

Uses the fog default if the availability zone is not passed, should
not break anything.

Tested locally with and without the availability_zone config.

1.2.1: Only skip provision with --no-provision

If --no-provision is not specify, the key provision_enabled does
not exist. So in order to skip, it needs to exist and be false.

Merge pull request #5 from fbouliane/release-1.2.0

Release 1.2.0

v1.1.14: Merge pull request #1 from simon-begin/pin-fog-xenserver

Depend on fog-openstack rather than fog