Do not bump follower term while it is isolated

Do not bump follower term while it is isolated

When a follower gets into a state that its isolated(ie cannot reach either peer)
it starts bumping its term every election timeout. Once the cluster heals it
would then disrupt the cluster because it has a higher term then the healthy part.
We can avoid this by not triggering new elections on followers that cannot
reach other peers while they are unreachable.

JIRA: CONTROLLER-1935
Change-Id: I8ee9f333740637ba5569287d405540f374bbc4bc
Signed-off-by: Tomas Cere <tomas.cere@pantheon.tech>