Upgrade to ES 7.2.1

[qreshi]

Issue #, if available: #109

Description of changes:
Upgrade to support Elasticsearch 7.2.1

Build output:

qreshi$ ./gradlew clean

> Configure project :alerting
=======================================
Elasticsearch Build Hamster says Hello!
  Gradle Version        : 5.5.1
  OS Info               : Mac OS X 10.14.6 (x86_64)
  JDK Version           : 12 (Oracle Corporation 12.0.2 [Java HotSpot(TM) 64-Bit Server VM 12.0.2+10])
  JAVA_HOME             : /Library/Java/JavaVirtualMachines/jdk-12.0.2.jdk/Contents/Home
  Random Testing Seed   : 7039F09BA248E56D
=======================================

Deprecated Gradle features were used in this build, making it incompatible with Gradle 6.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/5.5.1/userguide/command_line_interface.html#sec:command_line_warnings

BUILD SUCCESSFUL in 2s
4 actionable tasks: 4 executed



qreshi$ ./gradlew build

> Configure project :alerting
=======================================
Elasticsearch Build Hamster says Hello!
  Gradle Version        : 5.5.1
  OS Info               : Mac OS X 10.14.6 (x86_64)
  JDK Version           : 12 (Oracle Corporation 12.0.2 [Java HotSpot(TM) 64-Bit Server VM 12.0.2+10])
  JAVA_HOME             : /Library/Java/JavaVirtualMachines/jdk-12.0.2.jdk/Contents/Home
  Random Testing Seed   : 68569E23633D2C7A
=======================================

> Task :alerting-notification:compileJava
Note: /Users/qreshi/workspace/OpenDistro/Alerting/alerting/notification/src/main/java/com/amazon/opendistroforelasticsearch/alerting/destination/Notification.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.

> Task :alerting-notification:test
WARNING: An illegal reflective access operation has occurred
WARNING: Illegal reflective access by org.easymock.cglib.core.ReflectUtils$1 (file:/Users/qreshi/.gradle/caches/modules-2/files-2.1/org.easymock/easymock/4.0.1/26f1c39bd323b6ac7fe0c4c8e33bfdffcdde03a2/easymock-4.0.1.jar) to method java.lang.ClassLoader.defineClass(java.lang.String,byte[],int,int,java.security.ProtectionDomain)
WARNING: Please consider reporting this to the maintainers of org.easymock.cglib.core.ReflectUtils$1
WARNING: Use --illegal-access=warn to enable warnings of further illegal reflective access operations
WARNING: All illegal access operations will be denied in a future release

Deprecated Gradle features were used in this build, making it incompatible with Gradle 6.0.
Use '--warning-mode all' to show the individual deprecation warnings.
See https://docs.gradle.org/5.5.1/userguide/command_line_interface.html#sec:command_line_warnings

BUILD SUCCESSFUL in 3m 47s
50 actionable tasks: 49 executed, 1 up-to-date

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.