Add a warning in the microversion docs around the usage of 'latest'
Add a warning in the microversion docs around the usage of 'latest'

Per the mailing list thread on this subject:

http://lists.openstack.org/pipermail/openstack-dev/2015-August/073070.html

This adds a warning to nova's microversion docs on the usage of the
special 'latest' value.

Change-Id: Id78c0e7687e0a9deec271d1687589af304d948f5

TwinkleChawla/nova,zhimin711/nova,apporc/nova,cyx1231st/nova,cloudbase/nova,hanlind/nova,cernops/nova,mikalstill/nova,edulramirez/nova,j-carpentier/nova,j-carpentier/nova,MountainWei/nova,isyippee/nova,scripnichenko/nova,rahulunair/nova,mikalstill/nova,rajalokan/nova,yosshy/nova,sebrandon1/nova,Francis-Liu/animated-broccoli,edulramirez/nova,fnordahl/nova,felixma/nova,phenoxim/nova,dims/nova,mahak/nova,yosshy/nova,cernops/nova,NeCTAR-RC/nova,devendermishrajio/nova,mikalstill/nova,raildo/nova,JianyuWang/nova,scripnichenko/nova,sebrandon1/nova,gooddata/openstack-nova,vmturbo/nova,zhimin711/nova,rajalokan/nova,klmitch/nova,phenoxim/nova,bigswitch/nova,bigswitch/nova,klmitch/nova,vmturbo/nova,rajalokan/nova,rahulunair/nova,cernops/nova,rahulunair/nova,gooddata/openstack-nova,openstack/nova,CEG-FYP-OpenStack/scheduler,alaski/nova,cyx1231st/nova,rajalokan/nova,noironetworks/nova,jianghuaw/nova,Juniper/nova,Juniper/nova,felixma/nova,ted-gould/nova,CCI-MOC/nova,hanlind/nova,jianghuaw/nova,MountainWei/nova,alaski/nova,hanlind/nova,cloudbase/nova,jianghuaw/nova,BeyondTheClouds/nova,mahak/nova,noironetworks/nova,isyippee/nova,Juniper/nova,apporc/nova,openstack/nova,vmturbo/nova,ted-gould/nova,cloudbase/nova,sebrandon1/nova,devendermishrajio/nova,NeCTAR-RC/nova,dims/nova,klmitch/nova,Francis-Liu/animated-broccoli,jianghuaw/nova,openstack/nova,BeyondTheClouds/nova,raildo/nova,klmitch/nova,vmturbo/nova,JianyuWang/nova,TwinkleChawla/nova,gooddata/openstack-nova,CEG-FYP-OpenStack/scheduler,fnordahl/nova,BeyondTheClouds/nova,gooddata/openstack-nova,Juniper/nova,CCI-MOC/nova,mahak/nova