configuration to be able to remove x-krakend headers

[meshenka]

start of a potential fix for https://github.com/luraproject/lura/issues/727

Basically i add in core package a variable and wrap everywhere Krakend errors in a condition of this flag set at startup.

WDYT?

Next: well it definitely needs tests, it is just a Proof of Concept making sure i am in the righ track