Enhancements when regenerating keytabs and changing Kerberos configurations

In short, when regenerating keytabs or changing configuration in Kerberos tab in side bar, ambari-server will changes configurations which are different from ones in kerberos.json of stack.
It is caused by missing query parameter, config_update_policy=none. (refer to https://github.com/apache/ambari/blob/release-2.7.6/ambari-web/app/utils/ajax/ajax.js#L1985)
And also, I think it should allow administrators to choose whether to generate only missing keytabs when they change configuration in Kerberos tab. It should use regenerateKeytabs instead of restartServicesAfterRegenerate. (https://github.com/apache/ambari/blob/release-2.7.6/ambari-web/app/controllers/main/admin/kerberos.js#L575)

I will make a PR about this issue soon!

Related issues are:

• AMBARI-24229
• AMBARI-22699
• AMBARI-24319