Retry connecting to Heimdal if the first try fails

Retry connecting to Heimdal if the first try fails

In the Heimdal version of kadmin-backend, retry the kadmin connection
once if the first connection fails.  This is a workaround for a
transient networking error that we're seeing at Stanford and therefore
may not be fully appropriate for other sites.  Even on a successful
reconnect, this will cause some errors to be sent to standard error
due to the behavior of Heimdal::Kadm5.