This is a bundle of disparate changes that enable testing the
creating of a guest instance. At a high level these are:
* Add helpers for downloading ubuntu images.
* Add method for setting up a basic overcloud network. This is very
similar to zaza.configure.network.run_from_cli
* Add nova setup module that includes create_flavors and
manage_ssh_key
* Add nova test base class for launching an instance and two
derived classes for launching a cirros and lts image respectively.
* Fixes to zaza.charm_tests.test_utils after the refactor of the model
argument in zaza.models
* New certs.is_keys_valid function to check if a public and private
key are a pair.
* Methods in utilities.openstack for managing ssh keys. On creation of
a new key the private key is stored locally and the public key is
retrieved from Openstack
* Methods in utilities.openstack for testing guests at various points
of the creation process.
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