Allow to force auth challenge

[lmartelli]

I am trying to test a scenario where my auth challenge fails, but pebble does not always require a challenge (if one was successful before ?), so my test is not reliable.

It would be nice to have a config option to tell pebble to always do a challenge.

[mitar]

I think you can run PEBBLE_AUTHZREUSE=0 pebble.