Add example of encrypting field nested in an array

[ctrimble]

We recently had trouble encrypting a password nested under the /metrics/reporters path, using the provided instructions. This seems like a common use case and should be addressed by an example and an automated test.

We could run the following process:

• define which field was encrypted, using a mixin.
• encrypt the value, using the provided instructions, in a separate file.
• copy and paste the generated value into the configuration file.
• launch the app.

We could not:

• write the clear text value into the configuration file.
• generate an encrypted form of the configuration file using the clear text form of the configuration file and a JSON pointer to the field requiring encryption.