Pass omero.db.pass via envvar to bin/omero admin reindex

For security reasons, the DB password is not passed as an xargs to the reindex
command. This commit fixes this issue by internally setting the JAVA_OPTS
environment variable which should only be visible to the process owner (and
root).
Merge pull request #3161 from sbesson/cli_reindex_dbpass

Pass omero.db.pass via envvar to bin/omero admin reindex