add more explanation to the problem definition
add more explanation to the problem definition

This user story is somewhat controversial, in that some people still
believe it has no place in OpenStack.  So we need to make it clear
why this user story is needed, by giving more detail on the benefits
of bringing pets into OpenStack, and also explaining that the user story
can also benefit cattle.

This was also covered by this talk in Austin:

  https://youtu.be/lddtWUP_IKQ?t=3m6s

Change-Id: I0fa4687751cfb8043ea76bcaa86df3ffadf3c5d1

openstack/openstack-user-stories