Convert CPU_TRAITS_MAPPING to use os_traits
Convert CPU_TRAITS_MAPPING to use os_traits

Now that os_traits provides symbols for each trait, we can refer to
traits by symbol rather than by string.  This gives us compile-time
checking for free, in order to ensure that nova's use of traits lines
up with what os_traits provides.

Change-Id: Id1461c444f0f67b29e0a6a10181267ef1d1d8bc0

rahulunair/nova,klmitch/nova,klmitch/nova,rahulunair/nova,mahak/nova,mahak/nova,klmitch/nova,klmitch/nova,openstack/nova,openstack/nova,rahulunair/nova,openstack/nova,mahak/nova