Improve CLI: rally task list (of empty sequence)
Improve CLI: rally task list (of empty sequence)

If there is no tasks, print user friendly message
instead of showing empty table

Change-Id: I2c93bf11986ce04ce1eaed3c486db52eacf4887b

gluke77/rally,aplanas/rally,shdowofdeath/rally,vefimova/rally,vponomaryov/rally,cernops/rally,openstack/rally,varunarya10/rally,eayunstack/rally,gluke77/rally,vganapath/rally,pyKun/rally,yeming233/rally,group-policy/rally,vganapath/rally,pandeyop/rally,group-policy/rally,openstack/rally,eayunstack/rally,eayunstack/rally,cernops/rally,openstack/rally,go-bears/rally,gluke77/rally,pyKun/rally,amit0701/rally,paboldin/rally,shdowofdeath/rally,redhat-openstack/rally,paboldin/rally,paboldin/rally,amit0701/rally,aplanas/rally,varunarya10/rally,aforalee/RRally,ytsarev/rally,eonpatapon/rally,openstack/rally,vponomaryov/rally,pandeyop/rally,group-policy/rally,vganapath/rally,afaheem88/rally,gluke77/rally,vganapath/rally,aforalee/RRally,vefimova/rally,ytsarev/rally,redhat-openstack/rally,eonpatapon/rally,afaheem88/rally,yeming233/rally,go-bears/rally,amit0701/rally