Add support for backend config in spk infra scaffold

[yradsmikham]

Closes microsoft/bedrock#722 and microsoft/bedrock#727.

[andrebriggs]

@yradsmikham look good business logic wise. What is the plan to get the coverage greater than 44%?

[yradsmikham]

@andrebriggs, we are definitely keeping an eye on line and branch coverage while writing tests, and the hope is that scaffold would expand a little more in the coming sprints, which implies more unit testing.

If you look at the code coverage breakdown, you'll find that the reason why code coverage is "low" is because a lot of the code written for scaffold is reliant on file system (fs). There's certainly no issue with writing "mocks" for code that interacts with file system, but that starts to bridge the conversation around writing integration tests versus unit tests.

I find that code coverage statistics are something to consider when writing code, but does not always give an accurate depiction of code quality.

[jmspring]

Lgtm

[NathanielRose]

Running int a small thing where .backip still continues to get appended on an existing directory I am scaffolding. Not sure how we want to handle this in the future or note as a quick bug:

    Directory: C:\Users\naros\Desktop\Microsoft\fy20\spk\spk-infra\spk\discovery-service


Mode                LastWriteTime         Length Name
----                -------------         ------ ----
-a----       10/29/2019  11:14 AM           1137 definition.json
-a----       10/29/2019  10:45 AM           2033 main.tf
-a----       10/29/2019  10:45 AM            698 README.md
-a----       10/29/2019  10:45 AM            795 terraform.tfvars.backup.backup.backup
-a----       10/29/2019  10:45 AM           2670 variables.tf

[yradsmikham]

Running int a small thing where .backip still continues to get appended on an existing directory I am scaffolding. Not sure how we want to handle this in the future or note as a quick bug:

    Directory: C:\Users\naros\Desktop\Microsoft\fy20\spk\spk-infra\spk\discovery-service


Mode                LastWriteTime         Length Name
----                -------------         ------ ----
-a----       10/29/2019  11:14 AM           1137 definition.json
-a----       10/29/2019  10:45 AM           2033 main.tf
-a----       10/29/2019  10:45 AM            698 README.md
-a----       10/29/2019  10:45 AM            795 terraform.tfvars.backup.backup.backup
-a----       10/29/2019  10:45 AM           2670 variables.tf

This is resolved in this PR now :)