Enable an option where the plugin would decide on the VLAN tag for
a provider network. This is done as follows:
1. In the configuration file the admin will need to add the supported
DVS's and their respective VLAN ranges. For example:
network_vlan_ranges = dvs-22,dvs-70💯102,dvs-70:110:120
This means that dvs-22 can allocate any VLAN tag. On dvs-70
tags can be selected between and including 100 and 102 and
110 and 120.
2. When the admin created the provider network she/he need only state
the provider:physical_network (must be one defined above). If they
select a VLAN id then the selected one will be used. If not one from
the ranges above will be used.
Change-Id: Ieeebc790fa5a4e9480308dcd11f495662e4c48c2
8a3364c49f