Fix a bug in ex_get_console_output in the EC2 driver which would cause
an exception to be thrown if there was no console output for a particular
node.

Reported by Chris DeRamus.
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.