Add cli tests for existing TLS behaviour

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Rename host-based auth to identity-based auth

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

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Update cert-based auth docs to use new options

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

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Move default flag values to flags.go

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

Signed-off-by: Ben Firshman <ben@firshman.co.uk>
Prefix TLS options with --auth

Renamed --tlscert -> --auth-cert
Renamed --tlskey -> --auth-key
Renamed --tlscacert -> --auth-ca

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>