tc-testing: Restore original behaviour for namespaces in tdc

This patch restores the original behaviour for tdc prior to the
introduction of the plugin system, where the network namespace
functionality was split from the main script.

It introduces the concept of required plugins for testcases,
and will automatically load any plugin that isn't already
enabled when said plugin is required by even one testcase.

Additionally, the -n option for the nsPlugin is deprecated
so the default action is to make use of the namespaces.
Instead, we introduce -N to not use them, but still create
the veth pair.

buildebpfPlugin's -B option is also deprecated.

If a test cases requires the features of a specific plugin
in order to pass, it should instead include a new key/value
pair describing plugin interactions:

        "plugins": {
                "requires": "buildebpfPlugin"
        },

A test case can have more than one required plugin: a list
can be inserted as the value for 'requires'.

Signed-off-by: Lucas Bates <lucasb@mojatatu.com>
Acked-by: Davide Caratti <dcaratti@redhat.com>
Tested-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: David S. Miller <davem@davemloft.net>