Add https support

[Huang-lin]

No description provided.

[shafreeck]

I have some ideas about how to support https:

1. Pass scheme as part of address, for example use: http://192.168.0.1, http://192.168.0.2 or https://192.168.0.1, https://192.168.0.2 instead of current 192.168.0.1, 192.168.0.2
2. Add cetcd_setup_tls as you did to support custom cert files. The caller decide using it or not. The default trusted cert will be used if cetcd_setup_tls was not called.

This will be more simpler to implement.