Remove 'lconst' dependency on neutron.plugins.linuxbridge.common

This dependency is only used for pre-Havana versions of OpenStack.
Calico requires Icehouse or later, so we can replace the relevant code
by an error that announces this, and remove the dependency.
Remove processing of interface mappings

Processing of interface mappings, and the associated
'physical_interface_mappings' config entry, was inherited from the Linux
Bridge agent code.  In non-Calico OpenStack setups, interface mappings are
used to:

- give a name (e.g. "physnet1") to each subnet that one or more compute
  node is externally connected to

- specify, on each compute node, which of that node's network interfaces
  is connected to that subnet
  (e.g. "physical_interface_mappings = physnet1:eth0")

- specify how that subnet can be partitioned to transport OpenStack data,
  e.g. by using VLANs on it
  (e.g. "network_vlan_ranges = physnet1:100:299").

Calico doesn't need any of this:  (1) It doesn't do any L2 partitioning of
the external network, so doesn't need anything like network_vlan_ranges.
(2) It relies on normal IP routing, set up dynamically by BGP, to decide
which of the external subnets to use for transport OpenStack data.

In addition, and even in the Calico agent, "physical_interface_mappings" is
used - if configured - to generate a unique ID for the agent - one that
distinguishes it from other agents running on other compute nodes.  However
we haven't been configuring physical_interface_mappings in our setup, and
there is a fallback algorithm that generates the ID by enumerating all
Ethernet devices and taking the first one's MAC address.  We can continue
to rely on that fallback, as we already have been.

Therefore we can remove all of the interface_mappings processing.
Remove dependency on Linux Bridge agent config

The neutron.plugins.linuxbridge.common.config module executes code when it
is imported, so this isn't quite as simple as just removing the import
line: in addition we need to add code for the few bits of config setup
that we actually want, namely:

- the 'polling_interval' config option

- calls into neutron.agent.common.config to do common agent config to do
  with the root helper and agent state reporting.
RPM spec: remove dependency on linux bridge package
Merge pull request #9 from Metaswitch/linuxbridge-dep

Changes to remove unnecessary dependencies on linuxbridge code