Prevent excessive validation for maxPersonality limit

We have PERSONALITY property on OS::Nova::Server that defaults to {},
but code that triggers check for limits compares value of property
to None -> this property always get validated for limits (if limit is
set).

Change-Id: I552a03cd9f4ab570aa059349ab361bb50c9c0d1e
Closes-bug: #1333283
(cherry picked from commit 78b68ba0da5d6ad71397538e484fb7565333129b)
Merge "Prevent excessive validation for maxPersonality limit" into stable/icehouse