Fix bug 1533: use bionic as default series in proxy charms

Xenial is 2 LTS behind the current one. This patch will force bionic
rather than Xenial from now on. The current charms do not properly work
in Xenial.

The best solution will be to fix this bug:
  https://osm.etsi.org/bugzilla/show_bug.cgi?id=1551



Change-Id: I1a0c466b6151b19a1eaf5d97f0373ab271601f89
Signed-off-by: David Garcia <david.garcia@canonical.com>
(cherry picked from commit f643c135)