Add support for HTTP calls to 3scale backend

[mayorova]

Currently the plugin only supports HTTPS communication with 3scale Service Management API. SSL is preferred, but there might be cases, where it might be preferred to use plain HTTP, especially for 3scale AMP on-premises. For example, a real HTTPS certificate might not yet exist, and if the certificate on the backend is self-signed, currently the call also fails (which might be open as a separate issue).

The suggestion is to support both HTTP and HTTPS. Two possible options are:

• add a parameter which lets choosing HTTPS or HTTP, as it is done in the Ruby plugin
• support a full URL with schema which would define the protocol (http:// vs https://), as it is done in Python plugin (I personally prefer this one – less params, more flexibility)

[mikz]

Supporting full URI is definitely the way forward. Because you can minimise all params (host, port, scheme) into one. 👍