Added "git user accounts" to 'users' role

- 'users_git' is a list of user accounts configured to be used with
  git-shell over SSH connection. These accounts can be accessed only
  over ssh (command line access) and git (push, pull, clone);

- git-shell users have access to additional commands which can be used
  to create and manage bare git repositories on the account;

- there's a specific set of commands to enable deployment of web content
  from bare git repositories to separate set of directories in
  '/srv/www/' subdirectory;
Changed default sftponly /home directory

Default home directories for sftponly users will be created in
'/srv/users/sftponly' directory.