remove camel case naming in kerberos secret names

The names are currently used when HadoopKerberosKeytabResolverStep
tries to safe the kerberos delegation token into a kubernete secret.

However, the current camel case values will cause a
io.fabric8.kubernetes.client.KubernetesClientException
stating the following:

a DNS-1123 subdomain must consist of lower case alphanumeric characters,
'-' or '.', and must start and end with an alphanumeric character (e.g.
'example.com', regex used for validation is
'[a-z0-9]([-a-z0-9]*[a-z0-9])?(\.[a-z0-9]([-a-z0-9]*[a-z0-9])?)*')

Author: Rey Abe

resource-managers/kubernetes/core/src/main/scala/org/apache/spark/deploy/k8s/constants.scala

@@ -110,11 +110,11 @@ package object constants {
 
   // Kerberos Configuration
   private[spark] val KERBEROS_DELEGEGATION_TOKEN_SECRET_NAME =
-    "spark.kubernetes.kerberos.delegationTokenSecretName"
+    "spark.kubernetes.kerberos.delegation-token-secret-name"
   private[spark] val KERBEROS_KEYTAB_SECRET_NAME =
-    "spark.kubernetes.kerberos.keyTabSecretName"
+    "spark.kubernetes.kerberos.key-tab-secret-name"
   private[spark] val KERBEROS_KEYTAB_SECRET_KEY =
-    "spark.kubernetes.kerberos.keyTabSecretKey"
+    "spark.kubernetes.kerberos.key-tab-secret-key"
   private[spark] val KERBEROS_SECRET_LABEL_PREFIX =
     "hadoop-tokens"
   private[spark] val SPARK_HADOOP_PREFIX = "spark.hadoop."