ODP-2039: Set ZeppelinConf if not set

(cherry picked from commit 671e3b0)
ODP-2039: Read kerbros config from zeppelin-site.xml (if possible)

(cherry picked from commit 2519bdc)

Author: prabhjyotsingh

zeppelin-interpreter/src/main/java/org/apache/zeppelin/conf/ZeppelinConfiguration.java

@@ -98,6 +98,9 @@ private void loadXMLConfig(@Nullable String filename) throws ConfigurationExcept
     if (StringUtils.isBlank(filename)) {
       filename = ZEPPELIN_SITE_XML;
     }
+    if (!this.properties.isEmpty()) {
+      return;
+    }
     List<FileLocationStrategy> subs = Arrays.asList(
       new ZeppelinLocationStrategy(),
       new ClasspathLocationStrategy());

zeppelin-interpreter/src/main/java/org/apache/zeppelin/interpreter/InterpreterOption.java

@@ -92,6 +92,7 @@ public void setUserImpersonate(boolean userImpersonate) {
   }
 
   public InterpreterOption() {
+    this.zConf = ZeppelinConfiguration.load();
   }
 
   public InterpreterOption(String perUser, String perNote) {

zeppelin-zengine/src/main/java/org/apache/zeppelin/notebook/FileSystemStorage.java

@@ -57,10 +57,9 @@ public class FileSystemStorage {
   // your ticket expired.
   static {
     if (UserGroupInformation.isSecurityEnabled()) {
-      String keytab = ZeppelinConfiguration.getStaticString(
-          ZeppelinConfiguration.ConfVars.ZEPPELIN_SERVER_KERBEROS_KEYTAB);
-      String principal = ZeppelinConfiguration.getStaticString(
-          ZeppelinConfiguration.ConfVars.ZEPPELIN_SERVER_KERBEROS_PRINCIPAL);
+      ZeppelinConfiguration conf = ZeppelinConfiguration.load();
+      String keytab = conf.getString(ZeppelinConfiguration.ConfVars.ZEPPELIN_SERVER_KERBEROS_KEYTAB);
+      String principal = conf.getString(ZeppelinConfiguration.ConfVars.ZEPPELIN_SERVER_KERBEROS_PRINCIPAL);
       if (StringUtils.isBlank(keytab) || StringUtils.isBlank(principal)) {
         throw new RuntimeException("keytab and principal can not be empty, keytab: " + keytab
             + ", principal: " + principal);