Remove use of add_master.py.

saltstack v2014.7.0 has issues with the file_client-settings we left in
the minion-file, and in addition there is a bug which affects our use
of a list of masters (actually containing just one, but still causing
this problem: https://github.com/saltstack/salt/issues/17032
Parameterize saltstack version.
Introduce make full_provision.

The salt-provisioner in vagrant skips installation if there is a (as
in any) salt version installed. full_provision forces install.
Upgrade saltstack to v2014.7.0.

Release notes: http://docs.saltstack.com/en/latest/topics/releases/2014.7.0.html