Support operators (CRD based custom controllers)

[igorshevach]

Is your feature request related to a problem? Please describe.
Today CPA does not work with operators. instead, it keeps logging error "Error while autoscaling: failed to get managed resource: the server could not find the requested resource"

Describe the solution you'd like
Do what kubernetes HPA does; instead of querying replicas from a resource like replcationcontroller etc. retrieve scale object
(have a look at scaleForResourceMappings)

Describe alternatives you've considered
see above

[Jamie Thompson]

This is now available in version v2.5.0.