fixup network create : raise a nice neutron exception if entry already exist

Bad network request: ['default-domain', 'admin', 'NetworkA'] already exists with uuid: 759c7d73-b1b2-4296-8c6d-f460471c55c9

Signed-off-by: Nicolas PLANEL <nicolas.planel@enovance.com>
fixup network-delete() avoid flushing floating-ip-pool twice

fip_pool are flush already in virtual_network_delete()

Signed-off-by: Nicolas PLANEL <nicolas.planel@enovance.com>