promise/plugin: Implement check_service_state promise plugin

Add a promise plugin to check that the current state of a supervisord service matches the expected state.

This is step 2 of a 3-step plan to add an "autorun" feature to theia (See discussion in https://lab.nexedi.com/nexedi/slapos/merge_requests/888):

• Step 1: Add a slapos-node-auto service to slap/standalone: See https://lab.nexedi.com/nexedi/slapos.core/merge_requests/280
• Step 2: Create this generic promise plugin to check the state of a service.
• Step 3: Add an autorun option to software/theia with 3 possible values:
  • running: buildout starts slapos-node-auto and a promise is used to check that it is running
  • stopped: buildout stops slapos-node-auto and a promise is used to check that it is stopped
  • undefined: buildout leaves slapos-node-auto as is, the promise is always fulfilled.