Add an app validation check before start operation

[lucarin91]

Add a new validation step before starting a new app that does a set of static checks on the app.yaml (especially bricks). The test should include:

1. device check: we should be sure that all the devices required by the app are available on the board (now it is done here, which is too late)
2. bricks validation (implemented here:
   1. check if the brick exists
   2. check if all required variables are set

Additionally, we could consider some warnings, like when the user adds a variable that doesn't exist.