Switch pages configuration from command-line options to configuration file
Pages supports using a config-file for configuration. We should update omnibus so it uses this, instead of re-writing the
run script from a complex erb template.
An advantage to this approach is that we can make it possible to configure arbitrary values, so when we add a new feature to pages - like gitlab-pages!133 (merged) - it won't need us to come and add another option to omnibus!