Adding function to parse output of klist -k KeytabFile to get first principal

[Crefok]

Hey,
just found your repo and added a functionality to parse the given keytab file to get the first found principal if None is given.

Disclaimer, I was not able to test the code, because I couldn't get your tests with your provided image running. Maybe this will help someone anyway.

Thanks for you work!

[Crefok]

PS: this is just needed if there is no valid Kerberos Cache available. Else there is the static Method get_by_config to parse the principal. My provided Method could also be implemented at this Point of Code.