Override request method in the OpenStackBaseConnection and send a default
"Content-Type" header if "default_content_type" attribute is set.

This way, all the connection classes which inherit from this class don't
need to duplicate the logic and override request themselves.
Add initial implementation of a base networking API with a reference drivers
for OpenStack Neutron and OpenStack Quantum.

Also add support for retrieving a networking driver to the OpenStack compute
driver.

Note: Base API hasn't been decided upon yet and is subject to change.