Make Server compatible with Nova Key Pair resource

The Server resource checks that the specified key name exists in
validate(), but this will always fail if the specified key is created
with via a Nova Key Pair resource.

Make KeyPair.FnGetRefId() return None before the keypair is created so
that Server validation will not fail.  After the keypair is created,
FnGetRefId() will return the keypair name, as it did before this change.

Change-Id: I1cd6aa5c6555657c376df702754fd3d270ac44ec
Closes-Bug: #1271303
Merge "Make Server compatible with Nova Key Pair resource"