Support for config_drive in the Openstack driver

Adds a 'ex_config_drive' option to create_node and ex_rebuild_node
for the Openstack driver.

Closes #370

Signed-off-by: Tomaz Muraus <tomaz@apache.org>
LIBCLOUD-578: GCE adding Service Accounts to create_node

Closes #372

Signed-off-by: Tomaz Muraus <tomaz@apache.org>