Set govuk_provider to sky

We have some conditions for this Facter var. Matching them will allow us to
mimic real environments.
Set govuk_platform to staging

The conditionals currently around govuk_platform (though due to change) best
match the node definitions that we're parsing, such as IP addresses. Not
using production just in case it does point to real things.
Simplify additional Puppet shares

I missed the vendor/modules directory previously. However as a simpler
solution we can just share the entire parent puppet/ directory and then
overlay extdata/ on top of it. Vagrant will do the right thing.