add dualstack feature flags

[adobley]

What this PR does / why we need it

We have added four new feature flags for dualstack:
"features.management-cluster.dual-stack-ipv4-primary"
"features.management-cluster.dual-stack-ipv6-primary"
"features.workload-cluster.dual-stack-ipv4-primary"
"features.workload-cluster.dual-stack-ipv6-primary"

These features default to "false" and will need to be enabled for users
to use dualstack functionality while it is still under development.

Which issue(s) this PR fixes

Fixes #920

Describe testing done for PR

Tested combinations of the flag settings and TKG_IP_FAMILY configurations when running the tanzu commands to validate that the flags work as expected.

Release note

Experimental dual-stack functionality can be enabled with a feature flag.

PR Checklist

• Squash the commits into one or a small number of logical commits
  
• Use good commit messages
• Ensure PR contains terms all contributors can understand and links all contributors can access

Additional information

Special notes for your reviewer

[github-actions]

Cluster Generation A/B Results:
https://storage.googleapis.com/tkg-clustergen/1013/20211028233713/clustergen.diff.txt
Author/reviewers:
Please review to verify that the effects on the generated cluster configurations are exactly what the PR intended, and give a thumbs-up if so.

[github-actions]

Cluster Generation A/B Results:
https://storage.googleapis.com/tkg-clustergen/1013/20211029170846/clustergen.diff.txt
Author/reviewers:
Please review to verify that the effects on the generated cluster configurations are exactly what the PR intended, and give a thumbs-up if so.

[adobley]

Fixed a conflict.
@vmware-tanzu/tanzu-framework-reviewers Can someone take a look at this? It would be great to get this flag in.

[github-actions]

Cluster Generation A/B Results:
https://storage.googleapis.com/tkg-clustergen/1013/20211103175859/clustergen.diff.txt
Author/reviewers:
Please review to verify that the effects on the generated cluster configurations are exactly what the PR intended, and give a thumbs-up if so.

[vuil]

Thanks for the changes. I think the feature flag paths needs an update, but otherwise lgtm.

[github-actions]

Cluster Generation A/B Results:
https://storage.googleapis.com/tkg-clustergen/1013/20211105174323/clustergen.diff.txt
Author/reviewers:
Please review to verify that the effects on the generated cluster configurations are exactly what the PR intended, and give a thumbs-up if so.

[vuil]

lgtm, thanks!

[github-actions]

Cluster Generation A/B Results:
https://storage.googleapis.com/tkg-clustergen/1013/20211105184559/clustergen.diff.txt
Author/reviewers:
Please review to verify that the effects on the generated cluster configurations are exactly what the PR intended, and give a thumbs-up if so.