restapi: Create (or not) a vnic profile when adding network

The patch allows the user to override the default behavior
which is creating a default vnic profile for each network
added to the system.

The user will be able to create a network without a default
vnic profile by providing the following element in the network
POSTed element:

  <network>
      ...
      <profile_required>false</profile_required>
  </network>

See: http://www.ovirt.org/Features/Vnic_Profiles

Change-Id: I7f21fe627935a940db1a53da68ec8d55b2fb0973
Signed-off-by: Moti Asayag <masayag@redhat.com>