This is a small step towards making the running of PWD more efficient.
Building the parts of PWD outside of a container is always going to be
far more efficient than inside (and saves mapping of caches etc).
As a later step we could move entirely to this approach and use a
smaller image for running PWD.
Move away from using dep to use Go modules which is production ready as
of 1.14.
Also tidy up README to make instructions for local development clearer
Fixes#385
* Add Playground struct and basic support for creating it and retrieving
it
* Add missing functions in pwd mock
* Get playground from request domain and validate it exists. If valid set
it on the newly created session.
* Move playground specific configurations to the playground struct and use
it everytime we need that conf.
* Don't allow to specify a duration bigger that the allowed in the
playground
* Add DNS support for PWD instances
* Store IP address of PWD in all session networks and restore it with the
same IP address
* Remove unnecesary print
* Change url format to pwd<ip>-port for better DNS filtering
* Make PWD listen on 80 and 443 for DNS resolve to work
