Enable migration support in libvirt

QEMU breaks on ssh connections when the nova user has no login shell.
Since the bug was discovered during live migration, this log snippet
gives the error retrieved from nova:

2014-07-08 16:59:23.894 28674 ERROR nova.virt.libvirt.driver [-]
[instance: 61fe562c-7879-49c8-84b9-b100445414aa] Live Migration failure:
operation failed: Failed to connect to remote libvirt URI
qemu+ssh://nova@compute1/system?no_verify=1&keyfile=/etc/nova/ssh/nova_migration_key:
packet 1416128879 bytes received from server too large, want 16777216

This patch enables live migration when the nova user is set-up with
/sbin/nologin by enabling migration support in nova::compute::libvirt.
The puppet module sets the listen_tcp option to 1 whe migration is
enabled.

Closes bug 1340833

Change-Id: Ic78e631e1099dcc0c500eef2998b8a6c4f5d5f7b
(cherry picked from commit 34fe9b24d813d410ac34e021f71067190c52a1bd)