Support for base64 encode string keystore

[rahulnirgude]

Client applications use SSL/TLS to connect with Kafka brokers in order to implement secured communication. The clients initiate SSL communication with Kafka brokers using the SSL Engine constructed from the ssl.* properties pointing to key store and trust store. This PR addresses couple of important enhancements related to how the key store is loaded for secured communication with Kafka brokers.

Problem : Most of the container platforms such as PCF where the client applications are deployed set key store and trust store are environment variables with Base64 encoded PEM. Kafka clients expect the key store and trust store to be file system artefacts. This introduces custom logic to read these environment variables and create a key store / trust store out of that.

Solution : This can be solved by implementing loading the key store directly from environment variables as input stream. Two new configs ssl.keystore.as.string and ssl.truststore.as.string are added to indicate loading the key stores from these environment variables. When the values are true, it indicates the ssl.keystore.location and ssl.trustore.location are pointing to environment variables instead of paths.

Example configuration: ssl.truststore.as.string=true ssl.keystore.type=JKS ssl.truststore.location=${KEYSTORE} // populate this as an environment variable ssl.keystore.as.string=true ssl.truststore.type=JKS ssl.keystore.location= ${TRUSTSTORE} // populate this as an environment variable

All unit tests are passing and added a new test to verify that the base64 encoded string works with the code changes. No changes were made to any other unit tests.