Priority Label Manager in RM to manage application priority based on configuration

Focus of this JIRA is to have a centralized service to handle priority labels.

Support operations such as

• Add/Delete priority label to a specified queue
• Manage integer mapping associated with each priority label
• Support managing default priority label of a given queue
• Expose interface to RM to validate priority label

TO have simplified interface, Priority Manager will support only configuration file in contrast with admin cli and REST.