Automatic merge from submit-queue (batch tested with PRs 65582, 65480, 65310, 65644, 65645). If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Add Ubuntu 18.04 (Bionic) series to Juju charms

**What this PR does / why we need it**:
Adds Ubuntu 18.04 (Bionic) series to Juju charms. 16.04 (Xenial) is still the default series, but with this change the charms can be easily deployed on Bionic if desired.

**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes #

**Special notes for your reviewer**:

**Release note**:
```release-note
Add Ubuntu 18.04 (Bionic) series to Juju charms
```