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
- Renamed --tlscert -> --auth-cert
- Renamed --tlskey -> --auth-key
- Renamed --tlscacert -> --auth-ca
- Add DOCKER_AUTH_{CERT,KEY,CA} envvars for configuring cert auth.
- 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>
Make the default authentication type 'none' to match existing behavior

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)
Change allowed_hosts to known_hosts

Known hosts is a more familiar concept than allowed hosts for similar functionality.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)
Add deprecation warning to old tls flags

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)
Add auth flag to man page

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)
Change underscore to dash for key files

The authorized keys and known hosts default json file names now use a dash for consistency with other key files.

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)
Create public key file along with private key

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)
Add support for authorized key directory

Signed-off-by: Derek McGowan <derek@mcgstyle.net> (github: dmcgowan)