Added functionality to connect to a HTTP Basic Auth private registry

[robdewit]

I was having trouble connecting to v2 docker registry behind a proxy using HTTP Basic Auth. So this is my quick hack.

[yebinama]

Hello,
Thanks for the PR.
I have some questions about this functionality.

• Since you don't pass a token to Clair, how did you configure it to access to the registry?
• Instead of falling back to Http Basic Auth, maybe we could have 3 DockerRegistry Class: an abstract one, one for the Http Basic Auth authentication and one for the token authentication. The type of authentication should be set in the configuration file.