test/zdtm.py: add option to specify external criu for test suite

I am regularly running zdtm.py after updating the CRIU rpm to test if
the new rpm still works. Until know I usually did:

-criu_bin = "../criu/criu"
-crit_bin = "../crit/crit"
+criu_bin = "/usr/sbin/criu"
+crit_bin = "/usr/bin/crit"

This commit adds two arguments to zdtm.py:

  --criu-bin CRIU_BIN   Path to criu binary
  --crit-bin CRIT_BIN   Path to crit binary

It still defaults to the old values from above, but can now easily be
changed to the CRIU binary provided by the packaging system.

This change was more complicated than expected which is probably related
to the fact that zdtm restarts itself in namespaces.

v2:
 - rebase
v3:
 - fix errors reported by 'make lint'

Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>