Add bionic as supported distro
Add bionic as supported distro

Add the upcoming release Ubuntu 18.04 Bionic Beaver to the list of
supported distros. Drop the now unsupported 17.04 (zesty) instead.

Change-Id: Iea0b4bfdc510797f7886fac96eff6fdfb730252d

noironetworks/devstack,openstack-dev/devstack,openstack-dev/devstack,group-policy/devstack,dtroyer/devstack,noironetworks/devstack,group-policy/devstack,dtroyer/devstack,nawawi/openstack,nawawi/openstack