[Self Cert Tool] Enhance PreFlight checks to get actual count of PID limit on containers.

The Self Certification tool performs a couple of pre-flight tests to ensure that the underlying system is suitable for the Operator tests to run. 

These include checking number of processes(ulimit -u) and number of open files(ulimit -n).

These checks although sufficient doesn't provide an informative picture about the system apart from making sure the values are good enough for tests to run.

We should either enhance these checks or add new ones to highlight the thread limits of the container (eg: 1024 in Openshift containers) and/or other required parameters upon further discussion.