Allow compute manager to accept 3.0 messages

We are currently trying to support live upgrades from havana to
icehouse.  This change is required to make that happen.  In a live
upgrade situation, you have icehouse nova infrastructure, with a mix of
havana and icehouse compute nodes as you roll through and upgrade them.
While in this situation, the havana compute nodes must be able to accept
3.0 messages.

Accepting 3.0 messages is trivial to do and extremely low risk.  It does
not change the behavior of a havana-only deployment at all.  The only
additions is some pass-through code to the current compute manager.
This code will only get hit during a live upgrade situation.

Note that there is one other change needed to complete this.  With only
this patch, live upgrades are supported, but any operations that require
compute nodes to talk to each other will not work.  This includes
resizes and migrates.  That would be unfortunate, since live migrations
are something that people *definitely* want to be able to do during an
upgrade procedure.

The additional changes needed to allow resizes and migrates during live
upgrades are included in another patch.

Partial-bug: #1258253
Change-Id: Ia801609b6b3fe88e0d0b16f4c0d37bf25dbfa842
Merge "Allow compute manager to accept 3.0 messages" into stable/havana