OS-163: Reduce Cobalt API database calls.

Many database calls are avoided by using the instance references
provided by nova.db.sqlalchemy.api only once for each method.
Internal api and compute methods are modified to use references
instead of instance_uuid values (most of the time they use the
uuid to request a reference anyway).

These changes are more or less in line with how manager does
all its operations.

Change-Id: I83c458028545887cf7d128b6a4e48d4e4d0da416