In 1e6b5a4c004c520c5865c04c6bd3d40373d174c2 we've been a littl bit too
zealous and made lack of a keytab a fatal condition when a keytab was
specified explicitly in config files. However we distribute a config
file that explictly mentions the default keytab.

In general lack of a keytab is not really an impediment and gssproxy
will work properly until restarted (at which point saved cfredentials
will become unreadable as the encryption key is lost). But even if
access to crdential is lost in most cases this is a recoverable
situation.

So relax the constraint and fallback to a random key in all cases.