Writebackhost never set for Pelican origins outside of topology

[joereuss12]

As of now, writing is impossible to pelican origins outside of topology because writebackhost is never set. Starting a new Pelican origin does not tell the director how to write back to that origin, in addition the director doesn't have any way to tell the client how to write back either.

[joereuss12]

Update: a temporary fix is in place that the writebackhost is automatically set to the origin url. This should become configurable/set up with a flag when the user runs pelican origin serve

[joereuss12]

This is fixed with #622, Origins now have a boolean indicating whether they are writeable or not. In addition, instead of looking for the writebackhost for PUTs, we now get the origin_url directly and check if that origin is writeable.