Document the user Tomcat runs as, and how that affects log file ownership / permissions.

[kfogel]

The current SOLR-INSTALL.md file documents installing Tomcat to run as root. This is not ideal. However, it's not simply a matter of changing that one part in the documentation -- we also need to test the effect of running it as some other user (say, www-data) on the ownership or permissions of the log file(s) under /opt/apache-tomcat-6.0.35/logs/, so we can document the recommended ownership and perms for the logs too.