test_security.py: Ignore return code of grep.
test_security.py: Ignore return code of grep.

In the case that grep returns nothing, it also sets a return code
which will fail the test.  Ignore that by piping through cat and
just process the empty return string.

Changelog: Title
Signed-off-by: Drew Moseley <478ab6677b1bea404266cbf056683e99e5c5f252@northern.tech>

pasinskim/integration,pasinskim/integration,pasinskim/integration