This patchs aims at reducing the amout of code required by a collector for
metric validation (meaning no code at all in case the collector doesn't
require any additional metric collection parameters).
The following work has been done:
* Fixed the "check_configuration" method of the base collector and made it
return data in the same format as its child classes.
* Made each collector call the "check_configuration" method of the
base collector.
* Added some metric conf validation test cases
* Added details to docstrings in the base collector
Change-Id: Ib74f57602dd4413b9513dda7e161e02f1f60c619
45f0a00d56