Update ca-certificates by default when needed
Add support for empty user password in mysql role

When 'secret' directory is not defined or disabled, 'mysql' role will
create new user accounts with "" password (not empty, 'mysql -p'
argument still required to connect). Random passwords will be generated
when 'secret' directory is defined to allow storage and retrieval.
Add 'gitlab' role

- GitLab is an open source GitHub clone - http://gitlab.org/;

- 'gitlab' role installs and configures GitLab CE with nginx as
  webserver and MySQL as backend database. Support for PostgreSQL database
  will be added at a later date;

- 'gitlab' role is idempotent and should support future upgrades of
  GitLab CE (new releases every 22nd of the month);

- currently 'gitlab' role does not support all GitLab configuration
  options, support will be added as needed over time;