Fix `which salt-master` warning in Vagrant startup

The `which` command in Fedora 20 (differently from the one in Debian)
prints to stderr when the binary is not found. Redirect both stdout and
stderr to /dev/null to prevent messages from being printed by `which`.

Check whether the binary exists or not by the exit status of `which`
(non-zero means the binary does not exist) instead of checking for empty
output.

Tested:
- Started a Vagrant cluster with `vagrant up` and confirmed these
  messages were gone. Checked master and minions for Kubernetes
  components using the systemd status commands.

Fixes: Issue #1079

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Does >& work?