slap/standalone: add slapos-node-auto service

Add a slapos-node-auto service in slap/standalone that launches slapos-node-software and slapos-node-instance in a loop once per minute, and slapos-node-report once per hour (every 60 iterations).

This is step 1 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 this slapos-node-auto service to slap/standalone.
• Step 2: Create a generic promise plugin to check the state of a service (in slapos.toolbox)
• 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.