Add check network skill, start and stop skill

[fbrand-new]

This PR is a draft including the check network skill and the start and stop service skill.
What has been tested:

• Check network skill correctly returns not_connected when the the connection of the laptop is manually turned off and returns connected when it can actually ping an address (try with example.com)
• Start and stop services can trigger a node transition respectively from inactive to active and viceversa.

There are still many things to be done:

• Names of the services to start and stop are hardcoded.
• Only one service at the time can be stopped for now
• CheckNetwork has to be tested under multiple network loss conditions. Since (for now) it is a wrapper + parser of the ping utility there might still be unexpected behaviours.