Configurable grid subnet/supernet

[SpComb]

The kontena cli, server and agent originally used the hardcoded 10.81.0.0/19 subnet for the grid overlay network. With the kontena-IPAM in master, this is being expanded to 10.81.0.0/16 to allow the use of docker network create --ip-range for dynamic allocations (kontena/kontena-ipam#15) vs statically allocated host node addresses. With future support for multiple distinct networks, the kontena IPAM will currently use the default KONTENA_IPAM_SUPERNET=10.80.0.0/12 (kontena/kontena-ipam#17).

However, the grid subnet (10.81.0.0/16) in particular must not overlap with any other routes on the host nodes, which is an issue for platforms which lay claim to the entire 10.0.0.0/8 address space for infrastructure use, such as Packet:

For private addressing, we strongly reccommend using the 192.168.0.0/16 network for any shared back-end networks; the “10 network” (10.0.0.0/8) is used for numbering the physical interfaces on Packet bare-metal servers, as as well as shared services like iSCSI block storage, and is likely to cause collisions.

The solution for this is a per-grid configurable subnet (10.81.0.0/16) and supernet (10.80.0.0/12). These would be configured using kontena grid create --subnet 192.168.0.0/16 --supernet 172.16.0.0/12 testgrid, and presumably cannot be changed later

Note that any choice of grid subnet also comes with additional implications when using the Kontena VPN... e.g. using 192.168.0.1 for the initial kontena node address would be highly likely to cause issues when using the Kontena VPN..

Ideally this would also include support for IPv6 Unique Local Addresses (ULA), which can be automatically generated at grid creation time. However, this would require additional support for IPv6 within both the kontena-ipam (kontena/kontena-ipam#10) and the overlay network (weave).