Move checking/logging out of executeModification()

We will need specialized methods to deal with modifications,
similar to what we are doing for read(). As a first step, split
out common bits. This inlining has the added benefit of not touching
class reflection data.

JIRA: CONTROLLER-1950
Change-Id: I88030331ad8fe9299e8e824e18571047c87c90c6
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>