arm-pod1: Pin older kernel to avoid link issues

arm-pod1: Pin older kernel to avoid link issues

Current Ubuntu Trusty kernel `linux-image-generic-lts-xenial`,
backported from Xenial, introduced a regressiong with our
10G Extreme Networks switch - sometimes the link negotiation
fails.

This was not a problem with the previous 4.4 kernel we used,
4.4.0-45-generic (current is 4.4.0-47-generic), so pin it until
upstream fixes the rootcause.

Since the local mirror only contains the new kernel, bring back
access to public Ubuntu repos (only for this pod).

NOTE: Our admin interface is on a PCI NIC, so we will keep
using the new (faulty) kernel for bootstrap image.

See [1] for full problem description.

[1] https://jira.opnfv.org/browse/ARMBAND-168

Change-Id: I39505013804620f1b910169492e895a637d20de2
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>