Don't overwrite last relation key when username not found
When rotating a password, the code updates the password on the relation
bag for the associated relation. However, if the username wasn't found
in the relation data (e.g. if it was in app-data instead) then the code
unfortunately overwrote the last key it looked at (and this was,
randomly, private-address). This was due to a bug in the code. This
patch fixes that problem.
However, the charm (or at least certainly the rotating passwords code)
doesn't support app data bags as it doesn't find the matching username
to update the relation data. This means that it doesn't support rotating
passwords with charms that use app-data. This is the note added to the
README.
Add the 'docs' key and point it at a Discourse topic
previously populated with the charm's README contents.
When the new charm revision is released to the Charmhub,
this Discourse-based content will be displayed there. In
the absense of the this new key, the Charmhub's default
behaviour is to display the value of the charm's
'description' key.
