Make the user agent selection in the HTTP Requests test be static

This is to avoid false positives due to user agent based targeted content
Disable by default user agent randomization in HTTP Request template
Update user agent strings for user agent randomization

They are now taken from the PETS2010 paper "How unique is your browser" as the
user agents with the largest anonymity set.
Note: this is probably no longer true as of today, but I could not find a
better source for mostly used user agent strings and it's for sure better than
what we had before (We were considering highly used Opera 8.1).