Terratest: Basic usage test

[jlsan92]

This is the beginning of tests use cases using Terratest 👍

So far, the only test ensures that the module is deployable using basic settings (described in the basic example).

I've faced many issues learning Golang (and its module's resolution) and I'm not happy how the go test command looks like inside the TravisCI config, so probably some help would be great 🙏

EDIT: Thanks @honzahovorka @varholak-peter for the help with the go modules 🍻

Closes: #11

[jlsan92]

As expected, adding this basic test, some bugs were found 😄 but now they are fixed and included in this PR

1 - Internal Module AWS region: The AWS region is used in some policies/interpolation files and by default it was us-east-1 (exposed as a variable). Now the variable is set to "" and the region used internally is the same used by the provider invoking the module. If for some reason you need to use a different region, the variable region needs to be explicitly set now.

2 - ALB's public subnets: All the Public Subnets generated by the VPC module were attached to the Internet-facing Load Balancer but if there is at least more than one subnet repeating AZs, the ALB couldn't be created. This is the case for some regions like São Paulo 🇧🇷that (in this moment) only has 2 AZs. So now, the number of subnets attached to the ALB is less or equal to the number of AZs in the current region

👍