tendrl-node-agents that monitor other tendrl-node-agents

[r0h4n]

tendrl-node-agent provide's heartbeats to central store, however this requires external monitoring to ensure all node-agents are running.

Currently, tendrl node-agents running on the tendrl-api node are tagged with "tendrl/monitor" role.

Members of this role will monitor all tendrl-node-agents. In case of a dead or zombie node-agent, the monitoring node-agents will mark the same appropriately (i.e. "unavailable")

[r0h4n]

@brainfunked Need your inputs

@anmolbabu mentioned tendrl-performance-monitor ping's the tendrl managed nodes frequently, we need to figure out if the "tendrl/monitor" node-agent should monitor all nodes or tendrl-perfromance-monitoring should do that.