Refactor: get rid of local and no_leader and global variables

Thanks to the change with the former, attaching it to the now de facto
generalized "context" in struct booth_config being passed around, we
can now simplify find_myself (both underscore-prefixed and plain) so
that no extra outbound argument propagation is needed (argument is
dropped).

Signed-off-by: Jan Pokorný <jpokorny@redhat.com>