[#137374301] adds stage stemcell job to ert upgrade pipeline
[#137374301] adds stage stemcell job to ert upgrade pipeline

Signed-off-by: anwar chirakkattil <7cb5ef61da66b1dc08c9907356ef31750a339f41@pivotal.io>

BrianMMcClain/pcf-pipelines,sandyg1/pcf-pipelines,pivotal-cf/pcf-pipelines,sandyg1/pcf-pipelines,sandyg1/pcf-pipelines,cah-josephgeorge/pcf-pipelines,beibeiyang/pcf-pipelines,sandyg1/pcf-pipelines,mcb/pcf-pipelines,rahulkj/pcf-pipelines,pvsone/pcf-pipelines,rahulkj/pcf-pipelines,BrianMMcClain/pcf-pipelines,mcb/pcf-pipelines,pivotal-cf/pcf-pipelines,cah-josephgeorge/pcf-pipelines,beibeiyang/pcf-pipelines,rahulkj/pcf-pipelines,cah-josephgeorge/pcf-pipelines,pivotal-cf/pcf-pipelines,pvsone/pcf-pipelines,pivotal-cf/pcf-pipelines,pvsone/pcf-pipelines,rahulkj/pcf-pipelines,mcb/pcf-pipelines