This adds support for basic authentication which closes #3090

The support for specifying the client timeout was removed as discussed in https://github.com/prometheus/common/pull/123. Marathon was the only sd mechanism doing this and configuring the timeout is done through `Context`.

DC/OS uses a custom `Authorization` header for authenticating. This adds 2 new configuration properties to reflect this.

Existing configuration files that use the bearer token will no longer work. More work is required to make this backwards compatible.