install_rally.sh: Get latest pypi links more robustly
install_rally.sh: Get latest pypi links more robustly

This parses the JSON from PyPI with a real JSON parser rather than
trying to parse the HTML with shell tools, which should be more
failsafe going forward.

Change-Id: I3d2476428bc6da3a0b11a38e4fdd12c2d64272f4

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