Fix install_rally.sh to get it to work on MacOSX
Fix install_rally.sh to get it to work on MacOSX

On MacOSX, `mktemp` requires being passed a template. This change
modifies the calls to `mktemp` to explicitly pass a template so
that the code works on both MacOSX and linux.

Change-Id: Iacb6c07cf41a988d67bbd7877f4dcc2b369f4f81

amit0701/rally,amit0701/rally,vganapath/rally,eonpatapon/rally,eayunstack/rally,yeming233/rally,yeming233/rally,vganapath/rally,amit0701/rally,eayunstack/rally,vganapath/rally,openstack/rally,eayunstack/rally,openstack/rally,gluke77/rally,openstack/rally,openstack/rally,gluke77/rally,eonpatapon/rally,gluke77/rally,gluke77/rally,vganapath/rally