generic driver: support additional SSH options for service instance

Dear Manila developers,

thanks for your good work with the project, really appreciated.

This is a feature request. In our environment, SSH to virtual machines need to be proxyed using an intermediate SSH host.

When the generic driver creates a service instance, manila-share is unable to contact the VM because it wont use the SSH proxy.

I've read the source code and I found at least 3 places where this would need to be supported:

* on the utils module, the SSHPool class https://opendev.org/openstack/manila/src/branch/master/manila/utils.py#L105
* on the generic driver, the _ssh_exec() method https://opendev.org/openstack/manila/src/branch/master/manila/share/drivers/generic.py#L142
* on the service_instance module, the _test_server_connection() method https://opendev.org/openstack/manila/src/branch/master/manila/share/drivers/service_instance.py#L666

Moreover, the _test_server_connection() uses a raw connection to TCP/22. Assuming TCP/22 for SSH is in general fine, but perhaps it should be allowed to be configured.
Instead of a raw connection, I would suggest moving that logic somehow into the SSHPool class in the utils module, so if a proxy is configured, it can reuse such configuration for the connectivity test.

In summary I'm suggesting at least the following new configuration options:

* ssh_proxy_host = <FQDN, defaults to None>
* ssh_proxy_user = <String, defaults to None>
* ssh_tcp_port = <Integer, defaults to 22>

Then build a connection similar to what this SSH config would do:

* ProxyCommand ssh -a -W %h:%p <email address hidden>

I'm also open to follow other instructions to workaround this SSH limitation we're experiencing.