remove-machine gets wedged if you remove the primary out of 2

[John A Meinel]

If you do:
$ juju bootstrap
$ juju enable-ha -n3
# wait to be happy
$ juju remove-machine 2
# wait to be gone
# Juju will then be set up w/ 0 as the primary, and 1 as a non-voting secondary.
# doing
$ juju remove-machine 0

Will then wedge the controllers because it is trying to get 0 to be demoted as the primary, but it cannot, as 1 is not eligible to replace it. We need to update the code so that it first enlists machine 1 as a voting/eligible secondary and then asks machine 0 to step down.

[John A Meinel (jameinel)]

https://github.com/juju/juju/pull/8660