Add hab install to download and install required distros

[MHendricks]

Add the cli command hab install. This resolves the required distros for multiple URI's against a remote download server, downloads and installs any missing distros.

This also re-works how distros are resolved into a DistroFinder class or subclasses. These can be used to customize how distros are resolved by hab. This includes resolving distros from aws s3 buckets.

Checklist

• I have read the CONTRIBUTING.md document
• I formatted my changes with black
• I linted my changes with flake8
• I have added documentation regarding my changes where necessary
• Any pre-existing tests continue to pass
• Additional tests were made covering my changes

Types of Changes

• Bugfix (change that fixes an issue)
• New Feature (change that adds functionality)
• Documentation Update (if none of the other choices apply)

Proposed Changes

[MHendricks]

I've been thinking about dropping python 3.6 for a while and some tests I'm working on are suffering from packaging.requirements not comparing equal in 3.6 which is making it so I'm likely to drop official support for it in this PR.

[MHendricks]

I'm not dropping support for python 3.7 currently. It's still useful to be able to import hab in older applications.

However the hab install feature is not supporting using aws s3 for downloading. While it is still accessible boto3 has dropped support for it. If it stops working in the future I likely will drop support for it in python 3.7. The testing suite won't test the py37-s3 testing suite to check for issues.

[MHendricks]

I still need to update the readme but here is an example site file.

Adds the ability to resolve URI's and download and install any missing distros using a new cli command similar to pip install.
Example: hab install -u a/uri -u another/uri

The readme still needs updated but this example site file shows the basics of configuring hab install.

{
    "set": {
        "config_paths": [
            "{relative_root}/configs"
        ],
        "distro_paths": [
            "{relative_root}/distros/*"
        ],
        "downloads": {
            "cache_root": "{relative_root}/downloads",
            "distros": [
                [
                    "hab.distro_finders.s3_zip:DistroFinderS3Zip",
                    "s3://bucket-name/hab_distros",
                    {
                        "profile_name": "aws-credential-name"
                    }
                ]
            ],
            "install_root": "{relative_root}/distros"
        }
    }
}