ServiceNow plugins: Add mechanism for schedule conflict

[lrochette]

Add a mechanism to deny/wait when a conflict is detected.
Possible solution:

• add a parameter to drive the behaviour
  • name: CR_CONFLICT_POLICY
  • value:
    • ignore (default to keep current behaviour)
    • wait: Keep the flow running until the conflict is removed
    • deny: send a deny order to the pipeline

In addition, we will add an annotation to indicate why the pipeline is deny (Change request cancelled or conflict)