Add auth option to switch between auth methods

- Can switch between "identity", "cert" and "none"
- Replaced --insecure with --auth=none
- Moved logic to generate TLS configs to separate auth files
- Renamed --tlscert -> --auth-cert
- Renamed --tlskey -> --auth-key
- Renamed --tlscacert -> --auth-ca
- Added backwards compatibility for --tls... options. The existing tests
  for this have been left to ensure correct behaviour.

The DOCKER_CERT_PATH environment variable has been disabled for these
new options. Changing behaviour on whether files exist in the default
location (~/.docker/cert.pem, etc) seems dangerous if these files
are accidentally missing. This option was added in the first place to
make boot2docker work with cert auth, but now we have identity auth,
this is unnecessary.

Signed-off-by: Ben Firshman <ben@firshman.co.uk>