Improve database model.
Improve database model.

* add username property to NodeGroup
* hide private_key in Cluster
* remote is now a property of Instance

Change-Id: Iddcd0a3d2a17d71649e4c5a7ddcfd439993c8130

crobby/sahara,zhangjunli177/sahara,citrix-openstack-build/sahara,mapr/sahara,tellesnobrega/storm_plugin,henaras/sahara,ekasitk/sahara,zhujzhuo/Sahara,redhat-openstack/sahara,keedio/sahara,redhat-openstack/sahara,bigfootproject/sahara,openstack/sahara,esikachev/sahara-backup,keedio/sahara,zhujzhuo/Sahara,matips/iosr-2015,xme1226/sahara,zhangjunli177/sahara,mapr/sahara,rnirmal/savanna,tellesnobrega/storm_plugin,ekasitk/sahara,mapr/sahara,bigfootproject/sahara,citrix-openstack-build/sahara,crobby/sahara,esikachev/scenario,egafford/sahara,keedio/sahara,xme1226/sahara,egafford/sahara,zhangjunli177/sahara,redhat-openstack/sahara,citrix-openstack-build/sahara,openstack/sahara,matips/iosr-2015,esikachev/sahara-backup,tellesnobrega/storm_plugin,esikachev/sahara-backup,zhujzhuo/Sahara,rnirmal/savanna,esikachev/scenario,henaras/sahara,tellesnobrega/sahara,matips/iosr-2015,ekasitk/sahara,bigfootproject/sahara,crobby/sahara,henaras/sahara,xme1226/sahara,tellesnobrega/sahara,esikachev/scenario