Support parsing argument_specs.yml

[gionn]

Since the introduction of the role arguments validation, it may happen to define an argument like:

my_role_argument:
  type: str
  description: My mandatory role argument
  required: true

which usually doesn't need a default value set in defaults/main.yml, otherwise such argument validation won't work as expected (it will always succeeded because of being defined e.g. '').

This prevent ansible-doctor to properly render those variables, unless also meta/argument_specs.yml is used to discover the available vars.

[xoxys]

Thanks for your feedback! Makes sense to me, would you like to work on the implementation?

[gionn]

I could but will be great if you provide some guidance on where to slip that in