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.