Fix typo wrt KRB5CCNAME

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>
dnshook: Export KRB5CCNAME before nsupdate

The following in bash is not acceptable:

  # run="x=y date"
  # $run

Therefore, in case of Kerberos authentication, we export KRB5CCNAME
before invoking nsupdate.

Signed-off-by: Dimitris Aragiorgis <dimara@grnet.gr>