Override clone s3 bucket path (#487)

Override clone s3 bucket path

Add possibility to use a custom s3 bucket path for cloning a cluster
from an arbitrary bucket (e.g. from another k8s cluster). For that
a new config options is introduced `s3_wal_path`, that should point
to a location that spilo would understand.

manifests/complete-postgres-manifest.yaml

@@ -63,6 +63,7 @@ spec:
   #  uid: "efd12e58-5786-11e8-b5a7-06148230260c"
   #  cluster: "acid-batman"
   #  timestamp: "2017-12-19T12:40:33+01:00" # timezone required (offset relative to UTC, see RFC 3339 section 5.6)
+  #  s3_wal_path: "s3://custom/path/to/bucket"
   maintenanceWindows:
   - 01:00-06:00 #UTC
   - Sat:00:00-04:00