Configuration revamp m2

NOTE : this is a pull request with conflict resolutions. original pull request is : https://github.com/wso2-dev/andes/pull/168

Introduced new configuration manager class (singleton) based on apache commons composite configurations to load the config files during initialization.

New configuration properties and their default values are maintained as enums.

qpid configuration classes were moved to "org.wso2.andes.configuration.qpid" from "org.wso2.andes.server.configuration"

MQTT configurations moved into andes config file

Added "commons-jxpath" since it is required by "commons-configuration" to handle xpath queries.

Added meaningful comments to describe properties and their usage.

NOTE : There is a significant namespace refactor here. the qpid configuration classes were moved from "org.wso2.andes.server" package to the "org.wso2.andes.configuration.qpid" package. These sources are not modified by wso2, rather re-used.
shall we change this variable name, regardless the fact that it exist in moqutte code base?