'create' and 'update' now have --prefer-ipv6 flags which can be used
to set the value of the backend attribute. 'describe' displays the
value of the attribute.

Also corrected the spelling of the --tcp-ka-enabled flag in error messages.

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

New Feature Submissions:

• Does your submission pass tests?

kpfleming@kpfleming:~/src/fastly/cli$ make test TEST_ARGS="-run TestBackend ./pkg/commands/backend"
ok  	github.com/fastly/cli/pkg/commands/backend	1.211s

Changes to Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully run tests with your changes locally?

User Impact

• What is the user impact of this change?

Are there any considerations that need to be addressed for release?