Add the functionality to support `CreateOrMutateConfigMap` and `MutateConfigMap`.
* `CreateOrMutateConfigMap` will try to create a given ConfigMap object; if this ConfigMap
already exists, a new version of the resource will be retrieved from the server and a
mutator callback will be called on it. Then, an `Update` of the mutated object will be
performed. If there's a conflict during this `Update` operation, retry until no conflict
happens. On every retry the object is refreshed from the server to the latest version.
* `MutateConfigMap` will try to get the latest version of the ConfigMap from the server,
call the mutator callback and then try to `Update` the mutated object. If there's a
conflict during this `Update` operation, retry until no conflict happens. On every retry
the object is refreshed from the server to the latest version.
Add unit tests for `MutateConfigMap`
* One test checks that in case of no conflicts, the update of the
given ConfigMap happens without any issues.
* Another test mimics 5 consecutive CONFLICT responses when updating
the given ConfigMap, whereas the sixth try it will work.
