Solve two nginx concurrent vhosts issue

Summary:
Previously, this image added a new site vhost for Phabricator, and
removed the default vhost provided by nasqueron/nginx-php-fpm.

Instead, we overwrite the default with our own configuration.

This allows to avoid the two files to be present in the same image,
as Docker 1.3 to 1.8 doesn't seem to honour a rm command on an upper
layer than the actual file.

Fixes T546.

Test Plan:

docker build --tag=nasqueron/phabricator .
docker run --rm nasqueron/phabricator cat /etc/nginx/sites-enabled/default

Reviewers: dereckson

Subscribers: Nasqueron Docker deployment squad

Projects: Nasqueron Docker deployment squad

Maniphest Tasks: T546, T552

Differential Revision: http://devcentral.nasqueron.org/D34