Update description of Rebuild in server_concepts.rst
Update description of Rebuild in server_concepts.rst

Change-Id: I3c6e04886ef478b8d4a52e9b3d073c5fa7d78094

openstack/nova,gooddata/openstack-nova,klmitch/nova,mikalstill/nova,openstack/nova,phenoxim/nova,rahulunair/nova,mikalstill/nova,rahulunair/nova,mahak/nova,mikalstill/nova,mahak/nova,openstack/nova,klmitch/nova,gooddata/openstack-nova,phenoxim/nova,mahak/nova,gooddata/openstack-nova,klmitch/nova,klmitch/nova,gooddata/openstack-nova,rahulunair/nova