Change Management Adapter

[venkataramant]

This PR contains ChangeManagementAdapter concept.
Apart of using the "Github issues" to track deployment status of puppet applies on nodes. This new Adapter enables to use any change management systems like JIRA/SNOW/internal ticket systems with additional logic(that is specific to each company). It has a bunch of simplified functions that are integrated with hecklerd/main.go. Based upon return values hecklerd either continue to apply puppet changes on nodes or skip them.
[1. In case of not able to create a Change Request 2. In case there is freeze on code/config changes in PROD env]

It has 2 important things

1. CM Adapter and cm_agent_cli (part of this PR)
2. CM CLI provider (a CLI that will interact with JIRA/SNOW/interact ticket). Every company has their own logic of creating/ updating Change Requests and determining if there is code/config freeze or deployment holiday ... These type of logics will be part of CM CLI provider.CLI developer has to implement a set of functions that are exposed in CM Interface and provide responses in the format that are exposed in cm_models package.
3. hecklerd_conf.yaml has new config section that contains the path of CLI.