allow for upgrade of the precise kernel

we are getting kernel crashes in the OpenStack gate, to test
getting around this we'd like devstack to be able to upgrade
the precise kernel to the latest lts backported kernel.

default to off

Change-Id: I5d47aa8d15b1b1c0386a13b65022f6b8108c5c49
Merge "allow for upgrade of the precise kernel"
Merge branch 'master' of github.com:openstack-dev/devstack