Code cleanup in ResourcePluginManager.initialize and in TestResourcePluginManager

Some code cleanup is needed in ResourcePluginManager#initialize: 

• There's a big code block that initializes resource plugins, this should be extracted to a separate method.
• Exception handling could be simplified.

TestResourcePluginManager minor cleanup: 

• Not thrown exceptions could be deleted from method signatures
• verify(obj, times(1)).<method>() calls: times(1) parameter could be deleted as it is the default if verify(obj) is invoked without the times parameter.
• Some code exceeds the 80 character column limit.