--vlan is not the same as --vlan_start

the lib/puppet/provider/nova_network/nova_manage.rb provider was using
the --vlan option when it meant --vlan_start.  This caused "network
create" commands with num_networks > 1 to fail with "duplicate vlan"
errors.

Change-Id: Ibdca3b8de72b459eacf8b44b6350b9bc71310c1b
Closes-bug: #1362772