ignore failures to apt-get update calls
ignore failures to apt-get update calls

systems may have a ton of apt repos on them, many of which aren't
really needed. We'd rather take the updates we can, and not fail
on secondary repositories, especially as secondary repository fails
will stop us dead in our tracks.

Closes-Bug: #1259911

Change-Id: I08b19cd4d48fba3b4884fa4bc011cde451136594

openstack-dev/grenade,dtroyer/grenade,openstack-dev/grenade,dtroyer/grenade