docker: Don't enable networking as a side-effect of DEBUG=1

docker: Don't enable networking as a side-effect of DEBUG=1

When trying to debug problems with tests it is natural to set
DEBUG=1 when starting the docker environment. Unfortunately
this has a side-effect of enabling an eth0 network interface
in the container, which changes the operating environment of
the test suite. IOW tests with fail may suddenly start
working again if DEBUG=1 is set, due to changed network setup.

Add a separate NETWORK variable to allow enablement of
networking separately from DEBUG=1. This can be used in two
ways. To enable the default docker network backend

  make docker-test-build@fedora NETWORK=1

while to enable a specific network backend, eg join the network
associated with the container 'wibble':

  make docker-test-build@fedora NETWORK=container:wibble

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20170713144352.2212-1-berrange@redhat.com>
[Drop the superfluous second $(subst ...). - Fam]
Signed-off-by: Fam Zheng <famz@redhat.com>