don't use /opt/ paths in dcachesrm-gplazma.policy

There's still a place in the template file, where /opt is used, namely the
default for the kpwdPath property.

* Change the default value for the kpwdPath to "/etc/dcache/dcache.kpwd".

Actually I'd personally suggest introducing a new subdirectory like
/etc/dcache/access_control/
where access control related files should go into. This may include things like
grid-vorolemap, grid-mapfile, storage-authzdb or dcache.kpwd.
/etc/grid-security doesn't seem the right place for them, as they are not
necessarily grid-related.
But that could be done in another patch.

Signed-off-by: Christoph Anton Mitterer <mail@christoph.anton.mitterer.name>