Install libffi-dev needed by python-glanceclient

Note that there is also a patch open to do this in
prepare_node_tripleo in the infra/config tree, if/when that lands this
could be reverted.

I've also moved the apt-get update to the front of the script as a
prophylatic against archive skew.

Change-Id: I136d9028b3e13e0cadabea553edd31519c30b562
Merge "Install libffi-dev needed by python-glanceclient"