add bb-master role

* see documentation role in roles/bb-master/README.rst
add user role

this roles allows to create a non-priviledged user

after creating a user, `getent` module is used to get its GECOS record
so, for example, user's home directory is available for following
roles/tasks.
add more params to proxy config
add play for configuring nine.bb.net

* ensure worker account exists
* create a virtual environment for the build master
* install and configure build master
  * Buildbot -- branch 'master'
  * metabbotcfg -- branch 'nine'
* create supervisor service for the build master (buildmaster)
* install and configure nginx