Fix bugs in selenium_grid_hub template

Found a couple of bugs that were preventing me from using Selenium Grid Hub template.

First, the "app" user was hard-coded in deploy-selenium-grid-hub.rb; it was failing when trying to chown. Switched this to use app_user as defined in rubber.yml.

Secondly, startup.sh was breaking when setting the MaxPermSize - the Ruby variable literal was showing up in the command, rather than being interpreted. This was due to the line being wrapped in single quotes, rather than double quotes.