Choose which intent we want for Launch Request

[jperata]

Is your feature request related to a problem? Please describe.
LaunchRequest is an Amazon intent for starting the app, the Default Welcome Intent is usually the one that would assume that role but you can remove it and create a different one in Google. We should have a way to set what we want as a LaunchRequest during the construction of the VGA instance.

Describe the solution you'd like
Have another method in Virtual Google Assistant builder that takes a string with the Intent you want to use when you specify the LaunchRequest, the default if that parameter is not set will still be the Default Welcome Intent

Describe alternatives you've considered
Find out which is the LaunchRequest in someway.

Additional context
Once this is changed, a parameter should be added to skill-tester-ml to support it.

Relates to #17