Better Support for New Relic Server Monitoring

[ttmc]

• Tested the existing code for installing the New Relic agent on all AWS instances, and made some improvements. For example, the New Relic license key is now read from an environment variable named NEWRELIC_KEY and there's a helpful error message if that isn't set.
• Added documentation about what New Relic Server Monitoring is and how to use it to monitor a BigchainDB cluster on AWS.

[vrde]

you can just do:

import sys

and then in the code:

sys.exit('...')

it's simpler

[ttmc]

@vrde Thanks, I changed it to import sys and sys.exit('...