jjb/releng: fix partial image name matching

jjb/releng: fix partial image name matching

Docker image building script opnfv-docker.sh tries to remove existing
images before and after building. It simply uses grep to matching image
names, which may cause partial name matching problems.
Ex. opnfv/compass-tasks will match both opnfv/compass-tasks and
opnfv/compass-tasks-base, and may lead to accidentally removing of
images still in use when several building tasks run concurrently.

Error log:
https://build.opnfv.org/ci/job/compass-tasks-build-arm64-master/6/console

Change-Id: I1a83f801c5e0cc889cd9e17b8d1b4d895f1f68a9
Signed-off-by: Yibo Cai <yibo.cai@arm.com>