promote: abort it when become non-candidate

box.ctl.promote() bumps the term, makes the node a candidate, and
waits for the term outcome. The waiting used to be until there is
a leader elected or the node lost connection quorum or the term
was bumped again.

There was a bug that a node could hang in box.ctl.promote() even
when became a voter. It could happen if the quorum was still there
and a leader couldn't be elected in the current term at all. For
instance, others could have `election_mode='off'`.

The fix is to stop waiting for the term outcome if the node can't
win anyway.

NO_DOC=bugfix

(cherry picked from commit ab08dad9)