1086377: Next system check-in not displaying in RHEL 5.11

Call used to determine if rhsmcertd is on worked for RHEL 6 and 7,
but not RHEL 5. Subprocess in Python 2.4 did not have the check_output
method. Subprocess.call gives a consistent answer in all versions. 0 if pidof
has a process number returned and 1 if it does not.
Merge pull request #958 from candlepin/wottop/1086377

1086377: Next system check-in not displaying in RHEL 5.11