Enhancement to support ACL bootstrapping

[yardbirdsax]

Fixes #119

Functional Changes

• The consul-iam-policies module now includes permissions for reading / writing SSM parameters when a new variable named acl_store_type is set to "ssm".
• The consul-cluster module now includes an input variable named acl_store_type, which is used to indicate where created ACL tokens should be stored. Currently this only supports the value "ssm", but could support other methods in the future. This value is only used to pass on to the consul-iam-policies module.
• The example Packer file now includes a step to install the bash-commons library, which is used in cases where ACLs are enabled.
• Several new common bash functions were created to enable creation of ACL tokens and reading / writing of SSM parameters in support of enabling ACLs in created Consul clusters.
• The run-consul script now accepts a switch parameter --enable-acl, which if set will cause the script to generate a root ACL token upon cluster provision, and store it as a specially named SSM parameter. The bootstrap command is executed on only one of the server instances; other server instances and client instances will wait for the SSM parameter to become available, then use it to create their own agent tokens with minimal rights. (Note: while this currently only supports SSM, it was written internally in a way that future expansion to other stores would be possible.)
• The run-consul script now accepts an argument --acl-storage-type, which is required when the --enable-acl flag is set. This allows the user to select how the bootstrap process stores generated ACL tokens. Currently the only supported value is 'ssm'.

Testing / Refactoring related changes

• Multiple Terratest related functions have been refactored to support the testing of ACL enabled clusters.
• Several bash functions have been moved to the new consul-commons.sh file so as to make testing them independent of running the run-consul script easier.

Known Issues

• The enterprise tests are failing. I'm not sure how to test that since I don't have access to the enterprise binaries. Open to suggestions!
• I was not able to test in versions older than 1.9.2 because of configuration mismatches (i.e. the conf file included options that weren't compatible with those versions).

Test results: https://gist.github.com/yardbirdsax/e08efec4799c7e286ce093b5dfee89c3

[yardbirdsax]

I think I've incorporated everything requested, please let me know if there's anything else required here? Thanks!

[yardbirdsax]

Howdy, just wanted to ping here since I still have this PR outstanding. Was there anything else I need to do to get it closed out?

[hashicorp-cla]

All committers have signed the CLA.