Add defaults entry in molecule yaml schema

[gciavarrini]

It would be useful to have a defaults section in molecule.yml

Let’s consider the following platforms section in the molecule.yml file.

platforms:
  - name: name1
        location: westeurope 
        resourcegroup_name: resource_group_name_A
        vm_size: <vm_size>
        marketplace_image:
            … etc …
  - name: name2
        location: westeurope
        resourcegroup_name: resource_group_name_A
        vm_size: <vm_size>
        marketplace_image:
              … etc …

location, resourcegroup_name and vm_size are the same both in platforms.name1 and platforms.name2

Adding a new defaults section to molecule.yml schema can ease the definition process and reduce redundancy in the file.

Expected yaml schema can be:

defaults:
      # varname: var_value
      location: westeurope
      resourcegroup_name: resource_group_name_A
vm_size: Standard_B1s
platforms:
  - name: name1
        # location, resourcegroup_name, and vm_size
        # aren’t explicit defined, so use default values.
        marketplace_image:
               … etc …
  - name: name2
        marketplace_image:
               … etc …