Support for packer v1.7.0

[bwhaley]

Fixes #948.

Adds support for running packer init in Packer version >= v1.7.0. A few comments about this change:

• This should be backwards compatible with Packer < v1.7.0.
• All the example Packer template are converted to HCL. I've kept the JSON equivalents in the repo for now since the old format is still supported.
• Introduces a new dependency, hashicorp/go-version. Hopefully this is low impact/low overhead.
• It turns out that when downloading plugins, Packer will happily stomp all over itself, causing checksum problems and difficult-to-debug failure modes if multiple tests are running and using the same plugins (e.g. the hashicorp/amazon plugin) at about the same time. To mitigate this, Terratest will instruct Packer to download plugins to a temporary directory, then will clean up when finished. This behavior is the default, but users can set DisableTemporaryPluginPath to disable it if desired.
• The empty S3 bucket tests were unreasonably slow, so I lowered the file count from 10,000 to 1,000. Hopefully that should still be a sufficient quantity to remain an effective test.

All comments & feedback welcome!

[yorinasub17]

Had a few nits, but overall LGTM! You can choose to address the nits in this PR, or in a follow up PR.

Feel free to merge as is (assuming the build passes!)

[bwhaley]

Thanks for the review. I've incorporated the feedback. Relevant tests are passing, but I notice that TestGitRefChecks/GetCurrentRefReturnsLightTagValue is failing with:

 git_test.go:56: 
        	Error Trace:	git_test.go:56
        	Error:      	Not equal: 
        	            	expected: "v0.0.1-1-g58d3ea8"
        	            	actual  : "v0.0.1-1-g58d3ea8f"
        	            	
        	            	Diff:
        	            	--- Expected
        	            	+++ Actual
        	            	@@ -1 +1 @@
        	            	-v0.0.1-1-g58d3ea8
        	            	+v0.0.1-1-g58d3ea8f
        	Test:       	TestGitRefChecks/GetCurrentRefReturnsLightTagValue
    --- FAIL: TestGitRefChecks/GetCurrentRefReturnsLightTagValue (0.02s)

Is this a known testing error?

[yorinasub17]

Is this a known testing error?

I don't think this is a known error given that it passed recently. I think this might be a new error caused by overlapping git commit SHA prefixes that caused it to require another character to differentiate that one?

[yorinasub17]

Updates LGTM!

[bwhaley]

Thanks! Relevant tests are passing. Will submit a new PR to fix the GetCurrentRefReturnsLightTagValue test.

[brikis98]

Ben, it looks like a few of the Packer templates were deleted... But these were used on the Terratest home page as examples: https://terratest.gruntwork.io/#test-with-terratest. If you click "Packer" and the "build.json" tab, you get an error. Could you update those examples to use the HCL files instead?

[bwhaley]

Doh, I thought I'd found all the references. Opened #966 to fix.