Split kerberos data collection into separate plug-in

The krb5 data collection in the samba plug-in is misplaced. Since
this is collecting information about the system keytab move it
to its own plug-in that can be activated on any kerberos enabled
system.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>

Conflicts:
	sos/plugins/samba.py
Do not collect krb5.keytab

Avoid collecting kerberos keytabs to prevent unintended
information disclosures. Instead retrieve a list using the klist
command.

Also remove the duplicative collection of this information from
the ipa plug-in (which should only be collecting non-system
keytabs that are specific to the IPA tools).

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>

Conflicts:
	sos/plugins/ipa.py
	sos/plugins/krb5.py