Don't kill nova-bm-deploy-helper prematurally

This change ensure that 'nova-baremetal-deploy-helper' process is
not killed by stack.sh when USE_SCREEN=False

Change-Id: I84f6f3c3d09bf0cd0d4c5d94eb486a1f7d0b1d0f
Merge "Don't kill nova-bm-deploy-helper prematurally"