election: promote 'manual' bootstrap master

A cluster may consist of only voters and manual nodes. This means
on bootstrap nobody would be elected as a Raft leader
automatically to create the first snapshot and register the
others.

After the previous commit the manual nodes were preferred to be
bootstrap masters, but they couldn't do anything.

This patch makes 'manual' bootstrap master promote itself for one
term so as it could boot the cluster.

Closes #6018