feat(dashmate): add configuration for seed node

[thephez]

Expected Behavior

Dashmate could be used to set up seed nodes for DAPI.

Current Behavior

Seed nodes cannot be created with dashmate

Possible Solution

Per discussion on Slack, adding a seed node config for dashmate would allow infra to more easily run seed nodes. Then, dapi-client could use them to get the masternode list.

Alternatives Considered

Additional Context

Came up from a question about the availability of a seed for mainnet