tomcat-jsvc.service has TOMCAT_USER value hard-coded

+++ This bug was initially created as a clone of Bug #1341850 +++
Description of problem:
The TOMCAT_USER value should be configurable instead of always "tomcat". Looking at the way that the service unit executes the tomcat script, I see that the functions script is sourced in the preamble BEFORE the configuration is loaded, so even if we check the TOMCAT_USER variable and try and use it, it would always be empty. Could we move the configuration sourcing before we source functions to make it configurable?

Version-Release number of selected component (if applicable):
tomcat-8.0.32-5.fc23.noarch

How reproducible:
Always

Steps to Reproduce:
1. Set TOMCAT_USER="notTomcat" in tomcat.conf
2. Start service with jsvc: systemctl start tomcat-jsvc
3. Observe that the process owner is "tomcat" instead of "notTomcat"

Actual results:
Process owner is "tomcat"

Expected results:
Process owner is "notTomcat"

Additional info:
This issue was raised against RHEL in the bug here https://bugzilla.redhat.com/show_bug.cgi?id=1201409. I'll link it as well.