Add dnf package manager support in Rally install script
Add dnf package manager support in Rally install script

Newest RHEL based Linux distributions uses dnf
package manager as default.

Change-Id: I3cc4d15c69501ba5da1ce775f96fde3968423155

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