[Ingest Manager] Send long polling timeout to the agent

[nchaulet]

Description

Related to elastic/beats#21037

We should send the long polling timeout to the agent so it can be use it to set a proper timeout.

It should be in the config change action probably something like

id: 75c06f20-f2a6-11ea-be6c-157b32ff6ced
revision: 1
outputs:
  default:
    type: elasticsearch
    hosts:
      - 'http://localhost:9200'
agent:
  monitoring:
    enabled: true
    use_output: default
    logs: true
    metrics: true
+  checkin_timeout: 60000

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[blakerouse]

Being that we are also going to add the URL's to contact Fleet in the configuration, maybe we should include it in the same kibana block?

[jen-huang]

We use Fleet Server now. Closing.