Allow multiple mgmt networks

[tiago-amado]

The issue:
I have multiple lab servers where I use to run CLAB setups.
For some containers it is useful to have direct remote access while others can still have a local subnet.
E.g. a setup with 15xSROS/SRL nodes and 1xvSTC. The client on my laptop needs direct access to the STC server (exposing ports is not feasible because there are many required ports), so an external IP is needed (all other nodes can have a local subnet). The option is to configure all nodes with external IPs but I don't have a block of 16 available IPs in the lab subnet... E.g.:
_`mgmt:

network: vstc_lab

bridge: br0 ### maps to the existing Server management bridge

ipv4-subnet: 10.82.182.0/24 # ip range for the docker network => existing br0

ipv4-range: 10.82.182.176/28 # requires a contiguous block

`_

The idea
It would be nice if we could use a mgmt network for vSTC (the existing server mgmt br and subnet), and another local subnet for the other nodes.
E.g. we could specify the "clab_mgmt" local network as default (assumed by default by the nodes), and another one mapped to the existing server mgmt BR, using the external IP@ (for this we must configure explicitly that the node has to use this one, or configure a static IP@ from this subnet range).

Note: I gave the example of vSTC, but can be many other out there.