Stop noise from 'test -e' check'

[noelmcloughlin]

Salt prints noisy [ERROR ] messages to stdout when test -e <dir> prints to STDERR.

[ERROR   ] Command 'test -e /srv/formulas/openssh-formula' failed with return code: 1
[ERROR   ] retcode: 1
[ERROR   ] Command 'test -e /srv/formulas/packages-formula' failed with return code: 1
[ERROR   ] retcode: 1
[ERROR   ] Command 'test -e /srv/formulas/firewalld-formula' failed with return code: 1
[ERROR   ] retcode: 1
[ERROR   ] Command 'test -e /srv/formulas/etcd-formula' failed with return code: 1
[ERROR   ] retcode: 1

This PR redirects the output to /dev/null to get rid of the noise.