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
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"