Allow slapd path configuration in t_kdb.py

Allow slapd path configuration in t_kdb.py

The upstream OpenLDAP installs slapd in libexec, which is not
typically in the path.  Also, copying the binary can sometimes cause
it to fail; for instance, in the OpenCSW package,
/opt/csw/libexec/slapd is a script which chooses a binary based on the
system architecture and the path to the script.  Allow the test runner
to set the SLAPD environment variable to specify the slapd location
and avoid the copy.