CI regressions will block merges

[Curtis Hovey-Canonical]

mgz advised me to revise the syntax to avoid confusing the Juju bot

> Adding $$fixes-<lp-bug-number>$$ as a comment to the PR will signal it
> is allowed to test. The token can be in any comment and in any part

When a merge testing into stable or devel is blocked by a critical
regression, you can signal your branch fixes that regression with this
syntax
    __fixed-<bug-id>__

Juju bot is currently looking for these tokens in a comment to test
merges into master.
    __fixes-1347715__
    __fixes-1342725__

Merges into 1.20 are unblocked

I also added a token to force a test merge. There are cases where it
is critical to merge and there is not critical regression. Maybe you
need to merge into both 1.20 and master. When we judge the branch
needs to merge, add this token to a comment:
    __JFDI__

-- 
Curtis Hovey
Canonical Cloud Development and Operations
http://launchpad.net/~sinzui