core: Bad request error doesn't returns enough...

Bad request error doesn't returns enough details (cannot understand
from the error what was bad/missing in request)

Removing the notNull annotation from the managementIp field since
although that this is true for new PM definitions, it is not mandatory
for updates and should be define exactly as the secondary management IP
field.

Change-Id: I163166bb6d58c475c7e3361821e928bc3f7b3a47
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1013301
Signed-off-by: Eli Mesika <emesika@redhat.com>