In my opinion the best choise for default value of discover_pr_origion
is not specified(None value) like it is done for bitbucket scm.
Currently there is no way to disable this option for github scm.
Added false value as one of valid value for this option, like is done
for discover-pr-fork-strategy. In fact any of yaml specification false
implementation should work.
Change-Id: Ibc26d6b5c39b5455160ba5c84be99575b79fc102
4e15ad0536