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

[shumkov]

Dashmate itself doesn’t require any changes. The deployment tool needs to be updated. I will provide a PR soon.

[shumkov]

It's ready dashpay/dash-network-deploy#664. Now we need to test it on testnet first, and then deploy on mainnet