Removed unnecessary parantheses and fixed formation

Some place in Nova, return value has unnecessary parentheses,
or return keyword is not followed by a space when returning a value,
which is a bad format and might be ambigous. For example:

    return(1)
    return{
        'a': 1
    }

As noone would write returns like:

    return1
    return'(empty)'

In this patchset we get rid of this bad formation.
Also, I've added a hacking rule which helps to prevent this in future.

TrivialFix

Change-Id: I4ff85d0240bf8719188ebd06fc0e98a81c1d1203

HACKING.rst

@@ -69,6 +69,7 @@ Nova Specific Commandments
 - [N356] Enforce use of assertIs/assertIsNot
 - [N357] Use oslo_utils.uuidutils or uuidsentinel(in case of test cases) to
   generate UUID instead of uuid4().
+- [N358] Return must always be followed by a space when returning a value.
 
 Creating Unit Tests
 -------------------