Allow core reviewers to abandon and unabandon

A much awaited feature, this patch is the outcome of running:

find modules/openstack_project/files/gerrit/acls/ \
    doc/source/stackforge.rst -type f -exec sed -i -e \
    's/^\(.* \)label-Workflow .*\(group .*\)$/\0\n\1abandon = \2/' \
    -e 's/.*exclusiveGroup.*Workflow.*/\0 abandon/' \
    {} \;

Change-Id: I217a8689528d14c9143de5656db01549f263c3b4
Closes-Bug: #1248794
Merge "Allow core reviewers to abandon and unabandon"