Custom logging.yml location

Adding the ability to specify an alternate logging configuration file via system property. Documentation updated.

Closes #2044
I would add a new test to [LoggingConfigurationTests](https://github.com/elasticsearch/elasticsearch/blob/master/src/test/java/org/elasticsearch/common/logging/LoggingConfigurationTests.java#L43) , where you set the system property that points to a temporary file and verify that the custom file gets loaded, similarly to what we are doing [here](https://github.com/elasticsearch/elasticsearch/pull/7457/files#diff-624a9ea00073006531b98491cb416d54R44), just make sure you read the review comments there as I wouldn't use a temporary folder but just write the temp file within the `globalTempDir()`.