Custom logger.yml location

Adding the ability to specify an alternate logging configuration file via system property. Documentation updated.

Closes #2044
I initially had the tests within the LogConfigurationTests which extend from ElasticsearchTestCase. However, I am not able to test my fix due to the following exception enforced by the JUnit Rules:
     java.lang.AssertionError: System properties invariant violated.

I have this as a comment on the class.