Use daemon flag only when USE_SCREEN=False

This patch removes the daemon option, when devstack is started
with USE_SCREEN=True.

Change-Id: Ib3b1c0b2abf9111b767b3d557dacdc7a89a38f59
Merge "Use daemon flag only when USE_SCREEN=False"
Merge branch 'master' of https://github.com/openstack-dev/devstack