Disable nginx from Debian Backports by default

When nginx package from Debian Backports is installed, nginx daemon
starts but does not put it's PID in /var/run/nginx.pid (file is empty).
After that, system cannot control nginx process via /etc/init.d/nginx
script. After nginx process is killed manually and restarted, PID is
saved correctly in /var/run/nginx.pid and everything works as expected.

Because above bug does not show up with nginx from Debian Stable,
I assume it's present in a backported package. Until it's fixed, by
default ginas will use nginx package from Stable. You can re-enable it
for testing in 'apt' role via APT pinning mechanism.
Fix "missing ssl_ciphers" error in 'nginx' role
Merge pull request #62 from drybjed/bugfixes

Bug fixes related to 'nginx' role