updated deploy to take in alternative aws profile

[AvidDabbler]

Addressing issue #283
This is an attempt to add support to allow multiple profiles in deployment. The solution that I put together does not allow for the copying an pasting of credentials nor a local import of json file, but simply picks out an alternative profile in the .aws/credentials file. This is best practice anyway and will help prevent accidental publication of local files that may contain keys. This pull request only addresses the 'deploy' command. If this method is acceptable I could take a look at other areas where it may be used.

AWS Credential Doc -> https://docs.aws.amazon.com/sdk-for-javascript/v2/developer-guide/loading-node-credentials-shared.html

[landonreed]

Hi @AvidDabbler, in our use of mastarm at IBI Group, we have actually adopted the practice of just setting the env variable AWS_PROFILE in order to reference a non-default profile (which I think that docs link also mentions).

So, if you're running a mastarm command in the terminal, that would like:

AWS_PROFILE=my_aws_profile mastarm deploy
# or
AWS_PROFILE=my_aws_profile yarn deploy

We've also adopted the use of the npm package execa for writing bash scripts in javascript. If you are looking to write scripts that invoke mastarm, this might also be useful. The usage looks something like:

const execa = require('execa')
await execa('yarn', ['deploy', '--s3bucket', 'myBucket], { env: { 'my_aws_profile' } })

In light of that, I'm inclined to close this PR. Thoughts, @evansiroky?