docs: describe migration and other movement concepts
docs: describe migration and other movement concepts

Migration (including cold and live migration), resize and shelve
are three concepts at the API level that support three distinct
sets of use cases. These all involve movement of servers and
so provide opportunity for operational efficiency.

This patch adds a section to the server concepts that outlines
the purpose behind the actions and the operational efficiencies
that can be gained by them.

blueprint complete-todo-in-api-concept-doc

Change-Id: I6b9c6c179c1bdd9400abae7deb1918b370ddaefd

BeyondTheClouds/nova,dims/nova,BeyondTheClouds/nova,cloudbase/nova,vmturbo/nova,phenoxim/nova,bigswitch/nova,dims/nova,Juniper/nova,gooddata/openstack-nova,cernops/nova,mahak/nova,rahulunair/nova,rahulunair/nova,jianghuaw/nova,devendermishrajio/nova,cernops/nova,rajalokan/nova,klmitch/nova,hanlind/nova,openstack/nova,rahulunair/nova,alaski/nova,CEG-FYP-OpenStack/scheduler,Juniper/nova,klmitch/nova,vmturbo/nova,jianghuaw/nova,gooddata/openstack-nova,alaski/nova,vmturbo/nova,rajalokan/nova,apporc/nova,bigswitch/nova,sebrandon1/nova,Juniper/nova,gooddata/openstack-nova,mikalstill/nova,klmitch/nova,klmitch/nova,cloudbase/nova,devendermishrajio/nova,sebrandon1/nova,mikalstill/nova,NeCTAR-RC/nova,sebrandon1/nova,mahak/nova,cyx1231st/nova,rajalokan/nova,openstack/nova,cyx1231st/nova,cloudbase/nova,zhimin711/nova,phenoxim/nova,openstack/nova,NeCTAR-RC/nova,cernops/nova,rajalokan/nova,vmturbo/nova,apporc/nova,zhimin711/nova,hanlind/nova,jianghuaw/nova,gooddata/openstack-nova,Juniper/nova,mahak/nova,mikalstill/nova,CEG-FYP-OpenStack/scheduler,jianghuaw/nova,hanlind/nova,BeyondTheClouds/nova