HADOOP-13990. Document KMS usage of CredentialProvider API. Contributed by John Zhuge.

hadoop-common-project/hadoop-common/src/site/markdown/CredentialProviderAPI.md

@@ -100,6 +100,7 @@ In summary, first, provision the credentials into a provider then configure the
 |SSL Passwords        |FileBasedKeyStoresFactory leverages the credential provider API in order to resolve the SSL related passwords.|TODO|
 |HDFS                 |DFSUtil leverages Configuration.getPassword method to use the credential provider API and/or fallback to the clear text value stored in ssl-server.xml.|TODO|
 |YARN                 |WebAppUtils uptakes the use of the credential provider API through the new method on Configuration called getPassword. This provides an alternative to storing the passwords in clear text within the ssl-server.xml file while maintaining backward compatibility.|TODO|
+|KMS                  |Uses HttpServer2.loadSSLConfiguration that leverages Configuration.getPassword to read SSL related credentials. They may be resolved through Credential Provider and/or from the clear text in the config when allowed.|[KMS](../../hadoop-kms/index.html)|
 |AWS <br/> S3/S3A     |Uses Configuration.getPassword to get the S3 credentials. They may be resolved through the credential provider API or from the config for backward compatibility.|[AWS S3/S3A Usage](../../hadoop-aws/tools/hadoop-aws/index.html)|
 |Azure <br/> WASB     |Uses Configuration.getPassword to get the WASB credentials. They may be resolved through the credential provider API or from the config for backward compatibility.|[Azure WASB Usage](../../hadoop-azure/index.html)|
 |Azure <br/> ADLS     |Uses Configuration.getPassword to get the ADLS credentials. They may be resolved through the credential provider API or from the config for backward compatibility.|[Azure ADLS Usage](../../hadoop-azure-datalake/index.html)|

hadoop-common-project/hadoop-kms/src/site/markdown/index.md.vm

@@ -302,6 +302,9 @@ Configure `etc/hadoop/ssl-server.xml` with proper values, for example:
 </property>
 ```
 
+The SSL passwords can be secured by a credential provider. See
+[Credential Provider API](../../../hadoop-project-dist/hadoop-common/CredentialProviderAPI.html).
+
 You need to create an SSL certificate for the KMS. As the `kms` Unix user, using the Java `keytool` command to create the SSL certificate:
 
     $ keytool -genkey -alias jetty -keyalg RSA