add new unit tests for plugin

This commit adds a few unit tests for the gearman-plugin.
A few things were refactored from creating the tests.
1. move methods from GearmanPluginUtil to GearmanPluginConfig
2. removed a few unused methods.

Change-Id: I8ccd0000ff528867c66958294192818f35d83383
Merge "add new unit tests for plugin"