Fix bashish in bin/gerrit.sh

The Gerrit init script uses a bashism to detect the location of jstack.
Instead of calculating it by trimming $JAVA, use $JAVA_HOME to obtain
the location of 'jstack'.

Also honor for the environment variable $JSTACK.

Change-Id: I94f0c672492becd6cdcc07769a7c70b290d222d4
1 file changed