Fix dev-quickstart.md - use pip to install openstack clients

Not all systems will have openstack clients in repositories.
The fixed guide suggest using pip to get up to date packages
via installing catch-all 'python-openstackclient'.

Change-Id: I5a84d944d51ede963f827737e917dd17e9466804

docs/dev-quickstart.md

@@ -57,8 +57,7 @@ permitted on aufs when building docker images.
 
 Next, install the OpenStack python clients if they are not installed:
 
-    sudo yum install python-keystoneclient python-glanceclient \
-      python-novaclient python-heatclient python-neutronclient
+    sudo pip install -U python-openstackclient
 
 Finally stop libvirt on the host machine.  Only one copy of libvirt may be
 running at a time.