Modify RBD plugin to use a single ID and move the id and key into the secret

RBD plugin needs only a single ID to manage images and operations against a
pool, mentioned in the storage class. The current scheme of 2 IDs is hence not
needed and removed in this commit.

Further, unlike CephFS plugin, the RBD plugin splits the user id and the key
into the storage class and the secret respectively. Also the parameter name
for the key in the secret is noted in the storageclass making it a variant and
hampers usability/comprehension. This is also fixed by moving the id and the key
to the secret and not retaining the same in the storage class, like CephFS.

Fixes #270

Testing done:
- Basic PVC creation and mounting

Signed-off-by: ShyamsundarR <srangana@redhat.com>

examples/cephfs/secret.yaml

@@ -4,11 +4,11 @@ kind: Secret
 metadata:
   name: csi-cephfs-secret
   namespace: default
-data:
+stringData:
   # Required for statically provisioned volumes
-  userID: BASE64-ENCODED-VALUE
-  userKey: BASE64-ENCODED-VALUE
+  userID: <plaintext ID>
+  userKey: <Ceph auth key corresponding to ID above>
 
   # Required for dynamically provisioned volumes
-  adminID: BASE64-ENCODED-VALUE
-  adminKey: BASE64-ENCODED-VALUE
+  adminID: <plaintext ID>
+  adminKey: <Ceph auth key corresponding to ID above>