Document XTF properties (and equivavlent environment variables)

[mnovak1]

Names and usage of XTF properties are spread across the framework and they're mostly undocumented. Purpose of this issue is to provide list XTF properties and document its usage in README.MD or any other suitable place.

Maybe better option would be to create single java class with all property names and their documentation and all tooling would reference this class so the list would be updated automatically.

Found classed with static xtf properies:
JUnitConfig
BuildManagerConfig
OpenShiftConfig
XTFConfig
WaitingConfig

then there are dynamic properties parsed in:
Product
Image