Add python ABC metaclass to the os-brick Connectors

This is a fairly small change to os-brick, but it will help define a required API for each of the Connector objects.

Right now the base class InitiatorConnector defines 2 methods connect_volume and disconnect_volume. Each of the child classes need to implement those, but there is nothing that requires them to do so in code.

This BP intends to convert the InitiatorConnector to an ABC metaclass that will then define a contract for child classes that impose the requirement to implement those methods and any other common methods in the future that each Connector has to implement.