"tkey" system test: fix key ID processing

If ns1/setup.sh generates a key with ID 0, the "KEYID" token in ns1/named.conf.in will be replaced with an empty string, causing the following broken statement to appear in ns1/named.conf:

tkey-dhkey "server" ;

Such a statement triggers false positives for the tkey system test due to ns1 being unable to start with a broken configuration file. Fix by tweaking the regular expression used for removing leading zeros from the key ID, so that it removes at most 4 leading zeros.