An executor can handle multiple archs (an executor that talks with a k8s cluster
with multi arch nodes). Don't use a label for archs but a custom executor
field.
* Add the concept of executor groups and siblings executors
* Add the concept of dynamic executor: an executor in an executor group that
doesn't need to be manually deleted from the scheduler since the other sibling
executors will take care of cleaning up its pods.
* Remove external labels visibility from pod.
* Add functions to return the sibling executors and the executor group
* Delete pods of disappeared sibling executors
Simone Gotti