Fixes projectcontour#862
Updates projectcontour#881

- Make it clear that all three TLS params are required if you supply any in all three modes.
- Update `contour serve` variables to make it clear they're not client certs.
- Remove redundant test now that all three params are required.
- Fix a small linting thing in `cli.go` by adding comments to exported functions.
- Add secure-local Make target
- Add env var support for all the TLS flags.

Signed-off-by: Nick Young <ynick@vmware.com>

- Take TODO about tidying up the repo root dir
- use varargs correctly in `Dial()`
- other style changes
- rebase from current master

Signed-off-by: Nick Young <ynick@vmware.com>

Signed-off-by: Nick Young <ynick@vmware.com>