Add support for tls/tcp transport protocol for live migrations

[Giuseppe Petralia]

Right now charm supports only ssh as auth type for migration:

$ juju config nova-compute-kvm migration-auth-type
ssh

That means block migrations are happening over scp, which is not the best option for transferring large files.

Comparing scp with netcat for transferring files between hosts, we can see that netcat is 3 times faster than scp.

To give some numbers in a production cloud, we see scp transferring file at 250MB/s whether we can go up to 750 MB/s using netcat.

It will be beneficial if charm can supports tls libvirt transport for better transferring performances for block migrations [0] or at least tcp [1] if tls is not supported by nova.

0. https://libvirt.org/remote.html#transports

1. https://docs.openstack.org/nova/ussuri/configuration/config.html#libvirt.live_migration_uri