Make instance object tolerate isotime strings

Any time we get a db object over RPC, the datetime has been
serialized to an isotime string. We have other legacy code that
needs to tolerate this, and those hacks would need to be
replicated in the objects transition. Instead, make the instance
object type function tolerate isotime-formatted strings so that
we can more easily convert RPC-transited database objects
until we're fully converted, at which time we could remove the
tolerance. Luckily, the objects still end up with a datetime
internally, which means the rest of the code can stop needing to
handle strings and datetimes gracefully.

Related to blueprint unified-object-model

Change-Id: I132559d3541d19353de6acc36a2740185e564105
Merge "Make instance object tolerate isotime strings"