quantum: move the client to the correct path

As for the other services the QuantumClient should not have a "client"
package prefix.

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
client: unify the execute method in OpenStackClient

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
command: remove the now unused subclasses

Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>