Improve the text in install-guide
Improve the text in install-guide

Update the text in install-guide for better understanding.

Change-Id: I8338b2e2c9d66f36ebcc3aaa0acfc3db8c19ebe6
Closes-Bug: #1549845

tengqm/senlin-container,openstack/senlin,openstack/senlin,tengqm/senlin-container,stackforge/senlin,stackforge/senlin,openstack/senlin