Nova scheduler host manager overhaul

It appears that HostManager class in nova contains methods which are responsible for general host selection as well as methods which are specific for obtaining and storing host-specific and service-specific data. We propose to separate generic parts responsible for hosts handling and leave them in HostManager class while moving service-specific methods to ServiceProvider class. These ServiceProvider class can be used for acquiring, storing and releasing on demand host-specific data (i.e. computing capabilities, storage space etc.). In future this can be used as a basis for building a generic host manager and scheduler. Also as for now HostManager seems to be in need for refactoring, which can be done in the framework of this blueprint.