Remove workaround for salt-minion startup in vagrant/provision-minion.sh

The workaround was not needed, as salt-minion was always correctly
started in the Vagrant minion setup.

The issue reported in #270 was clearly specific do System V style init
scripts and will not affect systemd.

Also remove the inaccurate comment from provision-master.sh, since -X
was not even really in use there.

Tested:
- Performed 3 full `vagrant up` and `vagrant destroy -f` cycles with at
  least 3 minions and up to 6 minions in one case. Checked that
  salt-minion was up in each of the minions using a `systemctl status
  salt-minion` command.
- Started nginx on the cluster using cluster/kubecfg.sh, confirmed it
  was up with `list /pods` and confirmed it was reachable using wget on
  port 8080 of the minions.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Merge pull request #1113 from filbranden/vagrant_salt_master3

Remove workaround for salt-minion startup in vagrant/provision-minion.sh