Adding further review comments on my contributions upto now on andes component.

1. Adding method comments, logging errors before throwing, using meaningful variable names. 

2. Convert AndesConfigurationManager into a static class instead of a singleton one. 

3. Add constants to define common errors during parsing configurations. 

4. Use uniform method naming conventions in AndesConfigurationManager. 

5. Handle the rare scenario of having an invalid default value. This exception is now handled inline instead of throwing outside. A null value is returned if this happens. (Given that its our responsibility to define semantically and type-wise correct default values, this scenario should ideally never happen.)

NOTE : This is a rebased PR. Original PR is : https://github.com/wso2-dev/andes/pull/216