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.
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