add local network type and use by default for tenant networks

add local network type and use by default for tenant networks

Fixes bug 1045142.

This patch adds 'local' as a new value for provider:network_type,
supported by the openvswitch and linuxbridge plugins. Networks of this
type provide connectivity through a bridge for VMs and agents local to
the host, but no external connectivity. They do not require
provider:physical_network or provider:segmentation_id values. These
local networks are intended mainly to support single-box
zero-configuration testing (including quantum gating), but may have
other uses as well.

For openvswitch, the new OVS.tenant_network_type configuration
variable selects what type of networks are allocated as tenant
(i.e. non-provider) networks. It defaults to 'local', but must be
changed to 'vlan' or 'gre' for openvswitch tenant networks to have
external connectivity. The default value is intended to support
single-box zero-configuration testing without any need to allocate
physical network resources or configure bridges, and without requiring
the operating system to support OVS GRE tunneling. It can also be set
to 'none' to completely disable creation of tenant networks.

For linuxbridge, the new VLANS.tenant_network_type configuration
variable works similarly, with a value of 'vlan' supporting tenant
networks with external connectivity.

With either plugin, administrators can create provider local networks
by specifying "--provider:network_type local". Additionally, with
openvswitch, provider GRE networks can now be created by specifying
"--provider:network_type gre --provider:segmentation_id <tunnel-id>".

A corresponding devstack patch is available at
https://review.openstack.org/#/c/12456/. With this patch, the
openvswitch and linuxbridge plugins are by default configured to
support only local networks. A set of shell variables, documented in
stack.sh, can be set in localrc to configure remote connectivity,
including bridges/interfaces available for provider networks.

Change-Id: I2812548326141d2212d04f34d5448fb974d298e0