Reformat paas-docker nginx configuration files for vhosts

Summary:
Based on Etherpad, proxy_pass is put first to be directly visible.
Then, another block includes the configuration and add custom changes.

Excepted if there are several proxy block, the parameters can be
defined at location level.

Test Plan: nginx -t

Reviewers: DorianWinty

Reviewed By: DorianWinty

Differential Revision: https://devcentral.nasqueron.org/D2846