Xenial does not install python2 by default; ensure that python2
and minimal module dependencies are installed prior to trying
to execute any python2 based hooks.
Update amulet tests to drop shared-db relation for n-gateway.
This change also includes a resync of charm-helpers to pickup
new changes for systemd support.
Change-Id: Icef9679cd1d1c96e8aa87bcedcf9735a41af8d52
Closes-Bug: #1555422