Silence Gatling STDOUT logging

[chriskilding]

There is a small amount of logging that is done directly to STDOUT. See if we can make a more invasive change in the Gatling private API to stop that.

Options:

• Duplicate the offending Gatling results printer class in our code - then omit all the STDOUT log calls.
• Use http://projects.lidalia.org.uk/sysout-over-slf4j/index.html and intercept everything, then omit calls from Gatling.

[chriskilding]

This can only be started once custom logging reporters have been created on top of loadtest4j, which emulate the vast majority of what gatling currently prints to stdout.