Stub out the dot keys access globally for all rspec tests

[KrauseFx]

Slightly related to #1078, but separate task: In rspec tests we never ever want to access the original .keys file that's being used by actual fastlane.ci running

[armcburney]

Hey @KrauseFx,

Don't we already have the functionality defined here? A usage example in a test suite which requires environment variables can be found here.

Do you want this to be applied to all test suties? If so, I can do that.