Refs #518: Added `TracAdmin` help commands for product scope.

Patch by Olemis.

Examples:
{{{
 $ trac-admin $TRACENV product admin <prefix> help
 $ trac-admin $TRACENV product admin <prefix> help component
}}}

git-svn-id: https://svn.apache.org/repos/asf/bloodhound/trunk@1488070 13f79535-47bb-0310-9956-ffa450edef68