Make a few places tolerant of sys_meta being a dict

('cause we're fixin' that crap.)

Change-Id: I8314e0d52ec2ae800765f60ce58ce9b309d7d513
Fixes for the instance object

These were needed to make Instance actually work in a real situation
in nova-compute. Also, we can't use get() because we need that for
dict emulation, so fix that.

Change-Id: I5c1e0331f3ca4e0d275686f416e3c3637853f6b8
Make Start/Stop use Instance object

This passes unit tests except for three stubborn cells ones.
Damned cells.

Change-Id: I8c6e33d379633de68b0536e4ef111da20fa14a2b