Add environment variable replacement to GetStringValueOrDefault

[abbotware]

Description of changes:
Allows a config file to use $(VARIABLE) that will be replaced with environment variables at run-time.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[abbotware]

I only use config files.. but having to put 'relative pathing' (....\ notation) everywhere is painful. I would much rather just use a token that gets expanded at runtime

[abbotware]

I can confirm this works perfectly for me.. as long as the {varaible} token is not typically used in any existing parameter/config this should be fine for everyone else too. If somehow a token appeared, it would get replaced with an empty string as the environment variable won't exist

[normj]

I like the feature. I am worried that opening and closing brackets is not unique enough to be sure the intention was an environment variable. I'm wondering if we should follow MSBuild precedence and use $(VARIABLE) instead.

https://docs.microsoft.com/en-us/visualstudio/msbuild/how-to-use-environment-variables-in-a-build?view=vs-2017

Also can you add some unit tests for this feature? You should be able to write tests with by setting environment variable in the test, create a command and then call GetStringValueOrDefault().

[abbotware]

what the token is - doesnt matter - its a regex. I just need any form of token replacement so I don't have to make convoluted config files. I'll change to $(VARIABLE) and make some unit tests

[normj]

This has been released as part of version 3.1.0 of Amazon.ElasticBeanstalk.Tools. Thanks for the PR and patience to get this out.