Support arbitrary extra args in rax module

Add extra_create_args and extra_client_args to rax module to support passing
advanced configuration options to client instantiation and server create calls.
Addresses #6708 fill in missing keywords for ssh.py _communicate
Fix for bug related to async jid polling change plus a new test
Merge pull request #6707 from sivel/rax-extra-args

rax module: Support arbitrary extra args
Add an ec2 example to with_subelements