Fix docker snapshot job choosing-strategy

  - choosing-strategy should be "gerrit" to make the job to
    checkout the job from given GERRIT_REFSPEC instead of master
  - hopefully final follow-up correction to
    https://gerrit.onap.org/r/c/ci-management/+/116262

Issue-ID: CCSDK-3019
Signed-off-by: Lasse Kaihlavirta <l.kaihlavirt@partner.samsung.com>
Change-Id: I400895a68cb6cb1e89fed1e75c9751abba59cc2e
1 file changed