[nailgun] relation between network_groups and nodegroups DB tables should be M:M

[Aleksey Kasatkin]

Version: Fuel 6.0 and later.

Relation between network_groups and nodegroups DB tables is M:1.
But default admin network is shared between all environments and default node group of each environment uses it.
It cannot be addressed in DB now as relation is M:1. It is addressed algorithmically in Nailgun code so it is hard to follow, especially for new developer.

Relation between network_groups and nodegroups DB tables should be changed to M:M so address default admin network relations explicitly.

This could also become (or not become - it will depend on design decisions) a prerequisite for moving node groups out of environment where each node group will be able to have several sets of network parameters: one set of network parameters per environment (seems, CIDRs should be the same but IP ranges may vary for different environments within one node group).

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