add some unit tests for the os_server module
add some unit tests for the os_server module

This commit adds some unit tests for the `cloud.openstack.os_server`
module.  These tests exercise `_network_args` thoroughly and
`_create_server` lightly.

These tests will **fail** until ansible/ansible-modules-core#2275 lands.

To run the tests:

    pip install -r test-requirements.txt
    PYTHONPATH=$PWD py.test

Originally from ansible/ansible-modules-core@3387526bca1dd52751a3719c462a599c6c094bb9

thaim/ansible,thaim/ansible