Change MANAGEMENT_NETWORK to MANAGEMENT_NETWORK*S*

Before: MANAGEMENT_NETWORK was either 'ovirtmgmt' or 'rhevm',
        and the value was gotten from automake
After: MANAGEMENT_NETWORKS = ('ovirtmgmt', 'rhevm')

This change allows the following patch to solve the bug.

Bug-Url: https://bugzilla.redhat.com/1015009
Change-Id: I69a39b7887267d54d4dce27d4818d32a7cc0cd68
Signed-off-by: Assaf Muller <amuller@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/23182
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>