LWRP for newrelic.yml creation

Hi,

I need to support multiple Java applications running on distinct JVM on the same host on the New Relic Dashboard.

The java-agent recipe kinda assume that you will have only one application configured, will all the same configurations.

I've moved the newrelic.yml template generation into a LWRP, allowing me to generate multiples copies tweaked for specifics of my multiples apps (mainly app_name honestly) that can then be called from the shell script that lauch the java apps (see the README for an example).

The same mechanics could be used for exemple by the nodejs agent (in my PR #46).

I don't know if this change make sense to you, but I'm PR-ing it anyway in case it does since it's not that invasive and could be useful for others.