Remove a modprobe warning in new systems.

New modprobe give a warning if configuration files do not end with .conf.
Add .conf to /etc/modprobe.d/blacklist-bridge in order to avoid this
warning.

Signed-off-by: Frediano Ziglio <frediano.ziglio@citrix.com>
Merge pull request #1359 from freddy77/modprobe_warning

/etc/modprobe.d/blacklist-bridge filename should end in .conf