Add boolean convertor to cells sync_instances API

cells sync_instances API contains "deleted" as the parameter, and the
parameter should be boolean. There are the other boolean parameters
such as "on_shared_storage" of evacuate API, they can be passed with
either boolean or string(e.g. "True").
For the consistency of whole Nova APIs, this patch adds boolean conv-
ertors to cells sync_instances API.

Change-Id: I4776539cfd723f302d10350757e1c341384c9ba2
Closes-Bug: #1261287
Changed error message to match usage

InstanceDiskTypeTooSmall is not the proper error message, it should be
FlavorDiskTooSmall.

Change-Id: Id928015534161cba296c17114ef2d19d96e8781d
Merge "Changed error message to match usage"
Merge "Add boolean convertor to cells sync_instances API"