Update sample configs to include master_kdc

Where we have ATHENA.MIT.EDU stanzas in sample or test krb5.conf files
which define kdc entries, also define a master_kdc entry.  Remove
default_domain and v4_instance_convert entries in examples as they are
only needed for krb5/krb4 principal conversions.  In the krb5_conf.rst
example, remove enctype specifications as we don't want to encourage
their use when they aren't necessary, and remove a redundant
domain_realm entry.

ticket: 7901 (new)