Improve devstack install script
Improve devstack install script

Do not fail on some non-fatal errors:

 * user rally already exists
 * ~/.ssh directory already exists
 * iptables is not available

Change-Id: Ibc937592e40097720be42d6442ec1c78f04722db

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