Make ServiceNetMap support custom network names

Make ServiceNetMap support custom network names

The rework to support custom roles moved the service net map
into a nested stack, but the logic it contains fails to consider
operators may override the network names via *NetName parameters.

So add a map_replace of the ServiceNetMapDefaults so we maintain
the existing default service->network assignments, even when the
network name is changed.

Note that operators will still be expected to pass any ServiceNetMap
overrides with the "new" network name, e.g whatever *NetName specifies,
otherwise I think environment files could get very confusing.

Change-Id: I52018e19428e11d26650f0ea6ccee3ae02494ff8
Partial-Bug: #1651541