Fix missing wrap_db_error for Session.execute() method

We should add wrap_db_error for Session.execute() method, because
Query.update() method will call Session.execute() and it is not inside
Session.flush(), so exceptions from Query.update() wouldn't be wrapped.

Add test for Session.flush() wrapper
Add test for Session.execute() wrapper

Fixes bug 1107890

Change-Id: I96894e502f9f279999b61a1e66469e34a07a022d
Added sample tests to FlavorExtraSpecs API

Partially implements blueprint nova-api-samples
Fixes bug 1097131

Change-Id: I38a51e656cc9474e331d8f568d9e7fb6ba62ef42
Merge "Added sample tests to FlavorExtraSpecs API"
Merge "Fix missing wrap_db_error for Session.execute() method"