Introduce payload into domain model

[soul2zimate]

domain model misses notion of payload. current payload definitions are:

EAP 7 payload on jira is a list of issues with same fix version and target release.
example: fix version 7.1.1.GA ( must same target release 7.1.z.GA ?) all issues in https://issues.jboss.org/projects/JBEAP/versions/12332890
(issues of fix version 7.2.0.GA is also a valid payload, just it's not a maintained CP payload version)

EAP 6 payload in bugzilla is a list of BZs block same parent bug which is manually created by GSS.
example: all issues block https://bugzilla.redhat.com/show_bug.cgi?id=eap6415-payload

[wolfc]

Prototyping at https://github.com/wolfc/payload-report/tree/bugzilla/src/main/java/org/jboss/set/payload/report