library/ipatest.py: New validation tests: krb5.conf and ipa ping

The first validation test of the krb5.keytab is now done using the system
krb5.conf file. If this test failed, then the validation will be done with
the temporary krb5.conf file.

An additionally IPA test has been added. For now this is "ipa ping" as there
seems not to be a more comprehensive validation test for proper IPA
configuration.