Adds subnet data source, allows no secondary range in subnets; fixes #5

[angstwad]

This will fix #5, which causes errors when the secondary_ranges map does not define any secondary ranges. This keeps the subnets_secondary_ranges output, but we might consider removing it altogether, since it is capable of producing such useless output as:

Outputs:

secondary_ranges = [
    [
    ],
    [
    ],
    [
    ]
]

Removing the output is something I'm happy to do, if that's the direction we choose to go.

[morgante]

Thanks for the contribution!