Update OWNERS approvers and reviewers: cluster/juju

cluster/juju/OWNERS

-assignees:
-  - chuckbutler
-  - mbruzek
-  - marcoceppi
-  - castrojo
-owners:
-  - chuckbutler
-  - mbruzek
+approvers:
+- castrojo
+- chuckbutler
+- marcoceppi
+- mbruzek
+reviewers:
+- thockin
+- mikedanese
+- nikhiljindal
+- pmorie
+- eparis
+- jlowdermilk
+- jsafrane
+- spxtr
+- krousey
+- david-mcmahon
+- ivan4th

cluster/juju/bundles/OWNERS

+reviewers:
+- jlowdermilk

cluster/juju/layers/OWNERS

+reviewers:
+- jsafrane
+- david-mcmahon
+- ivan4th

cluster/juju/prereqs/OWNERS

+reviewers:
+- eparis
+- david-mcmahon