Allow externalisation of secrets

[sheu]

When running the replicator executable, the user is required to provide secrets in the consumer and producer properties file. So far I have identified 3 values which are:

1. password in sasl.jaas.config field
2. ssl.keystore.password
3. ssl.truststore.password

It will be great if these value can be read as environment variables which are then merged into their respective files. An example will be to have the following environment variables:

1. CONSUMER_SASL_JAAS_CONFIG
2. PRODUCER_SASL_JAAS_CONFIG
3. CONSUMER_SSL_KEYSTORE_PASSWORD
4. CONSUMER_SSL_TRUSTSTORE_PASSWORD
5. PRODUCER_SSL_TRUSTSTORE_PASSWORD
6. PRODUCER_SSL_KEYSTORE_PASSWORD

The motivation behind this is that the files when deploying replicator in kubernetes environment keeping the configuration as code, the secrets will not be leaked.