Support config to turn off the context. prefix for log event context attributes

[jbedell-newrelic]

This is the first part of item 2 here: #2961

Relavent spec PR: https://source.datanerd.us/agents/agent-specs/pull/834

For this step we will simply add the defined config to allow users to turn off the prefix if they choose to. The default will still be to prefix the attributes. That will be changed with a later story done during a break change release.

[Newrelic-Boomi]

https://new-relic.atlassian.net/browse/NR-615105