Switch to using spawn to properly treat errors during sync_state

Changes f1b9ac5a and 9f6ff7e3 were aimed at improving the robustness
of the sync_state method. However both missed switching from spawn_n
to spawn to properly deal with raised exceptions.

Change-Id: Ieda4a5ecca63de67d8a9757962e7e3383fd9c9ea
Related-bug: #1257514
Update common network type consts to same origin

This patch removes new definitions of common network type constants (TYPE_FLAT,
TYPE_LOCAL, etc.) and modifies uses of aforementioned constants to a common
place where constants are defined (neutron.plugins.common.constants). This
patch does not change values that are equal in value but different in name:
NETWORK_TYPE_FLAT vs TYPE_FLAT. A second changeset will be made to handle that
case.

Unit tests were modified as well when they referred to the constant.

Finally, the ovs agent code refers to the OVS plugin constants directly and
these had to be changed as well. A TODO flag was put in that file due to use
of another plugin specific constant.

Network types that were only defined in a single plugin, such as mellanox's
infiniband (IB) network type was not carried over to the common constants file.

Fixes-bug: #1196170

Change-Id: Ib6bfc8275496a24bf247946d177c734b62ae44bb
Nicira: Fix core_plugin path and update default values in README

The following patch fixes the core_plugin path in the README file
which broke during the rename of quantum to neutron here ee3fe4e8.
This patch also updates a few of the default values and removes
the promise of adding support for multiple control clusters.

Issue reported by: Ben Lin <blin@nicira.com>

Change-Id: Ia46d80384efaa220685f98afa591177d00021362
Closes-bug: #1260550
Merge "Switch to using spawn to properly treat errors during sync_state"
Merge "Update common network type consts to same origin"