Lower log level of errors due to user requests to INFO

Errors due to bad client requests (e.g., NotFound, BadRequest)
are logged as exception/trace level and it is annoying
from the point of operators' view.
This commit changes the log level for errors due to
user requests (HTTP 4xx errors) to INFO.

Closes-Bug: #1282394
Change-Id: Ic5646333db88ce856f9366e914cf961890d30501
Rename DB models and related resources for VMware NSX plugin

This patch also introduces a DB migration to update the
affected tables, and the enum type for those DB engines that
require it.

Additions made to existing migrations are needed to ensure
that green deployments using the new plugin name will apply
the migrations correctly.

Partial-implements blueprint nicira-plugin-renaming

Change-Id: Ie0fdb51dfa399c44b749fa8345f334a2c8c29151
Merge "Lower log level of errors due to user requests to INFO"
Merge "Rename DB models and related resources for VMware NSX plugin"