Make bootstrap option '-bootstrap=n', where n is a number >= 3.

This actually gives us an option where we can have the bootstrap node
automatically unbootstrap itself after 'n' server nodes are up, which
means a leader node can be chosen automatically.

Default is -bootstrap=0 (same as -bootstrap=false). Doing
-bootstrap=1 or -bootstrap=2 enables bootstrap mode but does not enable
automatic unbootstrapping (so old -bootstrap=true behaviour).

Signed-off-by: Robert Xu <robxu9@gmail.com>