PUP-3255 improve acceptance pre suite

fix git install options
fix redhat json install issues
fix git url#branch generation

helps get machines up quickly when manual testing or writing tests.
might impact CI?
You could also argue that our CI system brings up machines and runs acceptance all day every day :smile:

The issue I have with removing rubygem-json from `:redhat` here is that the is a dependency of the Puppet RPM package. In fact, we vendor it and maintain it in our official Puppet Labs Yum repos: http://yum.puppetlabs.com/el/6/dependencies/x86_64/. From what I can tell, changing this to install from gem on EL only creates another difference between testing from Git and testing from packages.

To be fair, we do default to true for `add_el_extras` in acceptance/Rakefile:
https://github.com/puppetlabs/puppet/blob/e2549ac5f2f6e4507eac394ac5f7def4a68db9d4/acceptance/Rakefile#L27
So any EL-based distro (master or agent) should automatically have EPEL installed and available.

So this change is _probably_ okay, but at least worth considering.