Installation tests

[pippolo84]

The purpose of this PR is to introduce some tests to verify the registering and installation process of arduino-connector, without interacting with the real AWS API endpoints.

Regarding the TestRegister test, the following approach is used:

• create and install a custom Certificate Authority
• generate a server certificate for mosquitto
• generate a client certificate to subscribe to mosquitto and check what arduino-connector is publishing
• configure mosquitto to reject unauthenticated connections
• mock the OAuth server to be able to verify the token-based authentication flow
• mock the AWS API servers, to be able to check the Certificate Signing Request and send back a certificate built for this test
• run the register function
• check that arduino-connector is:
  • connecting to the broker using the generated certificate
  • publishing the correct information to the /register topic

[guerinoni]

LGTM