(PDB-468) Update scripts to use correct JAVA_BIN

The systemd service file and some of the other utility scripts were
using a hard-coded path to /usr/bin/java, instead of the java path
set in the system defaults file.

This commit updates those scripts to source the defaults file where
available, and fall back to sane defaults otherwise.
(PDB-468) Break out script common environment to LIBEXECDIR
(PDB-468) Update el spec file for added environment file
(PDB-142) Update puppetdb-foreground to respect JAVA_ARGS
(PDB-468) Change back to default java_args in non-persistent scripts
Merge pull request #864 from shrug/bug/pdb-468/1.6.x

(PDB-468) Update scripts to use correct JAVA_BIN