Currently container instances are hard-coded to join a single network,
the network associated with the session.
This change allows the caller of CreateInstance to specify which
additional networks should be joined. This is useful, for example, when
a container instance requires access to additional "backend" services
that may be running.
There are security implications associated with this change, hence the
additional networks are only joined when a new -unsafe flag is
specified. It is hoped the name is a sufficient indicator that thought
needs to go into using it.
* 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
