Set mandatory fields on RAD Lab UI via UIMeta flag

**Is your feature request related to a problem? Please describe.**
Currently the Fields are marked mandatory based on 2 different parameters:
1) There is no default value in variables.tf file
2) Some preset mandatory fields on the templates.

**Describe the solution you'd like**
Create a new UIMeta flag which will determine if the input variable will be mandatory or not. This is also a good time to think about the `depends_on` flag where a variable needs to be mandatory depending on another variable set.