Allow default configuration values in BeejuCore to be overridden

[massdosage]

Story
As a user of BeeJU
I would like to be able to override the default configuration values
So that I can customise certain values to my own setup.

Background
Sometimes we use BeeJU alongside HiveRunner and need to use HiveRunner's HiveConf values from BeeJU so that they both refer to the same metastore database etc. BeeJU currently allows a custom Map<String, String> configuration to be passed in but any values set in this are overridden by BeeJU's own defaults. The user should be able to provide both "pre" and "post" configuration maps so that they can specify configuration they want BeeJU to override as well as configuration that they want to override BeeJU's own.