Protect 'default_server' configuration

These changes should prevent change of the default_server if one has
been already saved in Ansible local facts by debops.nginx role. Default
server from facts should take precedence over any calculated ones and if
one is already present on the host, new one shouldn't be configured.
Reset nginx configuration if no local facts saved

If no Ansible local facts are present for nginx, this means that either
role configures nginx for the first time, or configuration is being
reset from a bad state. Either way, remove all symlinks from
/etc/nginx/sites-enabled/ to prevent accidental errors during Ansible
run from older, not yet regenerated nginx configuration files.
Merge pull request #9 from drybjed/default-server-fixes

Fixes related to 'default_server' option