ganeti-masterd: Master voting in separate process

One shouldn't fork a Python process after using threads. Master
voting is done before forking (utils.Daemonize), but it also uses
threads. Hence it's now called from a separate process.

This patch also fixes the check function to actually exit if
the current machine is not the master (introduced in 04ccf5e9).

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>