prevent prometheus sts update loop
prevent prometheus sts update loop

Changes in kubernetes v1.17 cause an endless update loop due
to validation errors cause by setting the 'apiVersion' and
'kind' fields in the StatefulSet spec.  These two fields are
set to empty strings by the Kube API server, and v1.17 added
validation that does not allow these fields to be modified.

See #2950

coreos/prometheus-operator,coreos/prometheus-operator