Fix web-server recipe order

This fixes a problem for some users who had to run `reconfigure` twice
to get correct permissions on `/var/opt/gitlab/gitlab-rails/uploads`,
because the `gitlab-www` group did not exist when the directory was
created the first time.
Make /var/opt/gitlab/nginx gitlab-www's home dir

This stops Chef from creating /home/gitlab-www on Centos 6.
Remove unneeded  write privileges from gitlab-www
Add 7.3.1 CHANGELOG entries
Merge branch 'nginx-fixes' into 'master'

NGINX fixes for 7.3

See merge request !218
Stop nginx when installing omnibus-gitlab 7.3.1

This is needed because we cannot change the home directory of gitlab-www
for people who are upgrading from 7.3.0 otherwise.