More informative nova-scheduler log after NoValidHost is caught.

This patch adds a warning log entry for when a NoValidHost
exception causes a scheduling error. The NoValidHost
exception's message will be logged before reporting that
the machine went to error. Previously, all other exceptions
would show a stack trace. However, since the NoValidHost case
is skipped, a user may not understand why his/her machine
went to an ERROR state. This patch seeks to remove any
confusion.

Change-Id: I0f01b1095251f5c659fe010c7376f3f6bc56babe
Closes-Bug: 1336977
Remove duplicated code in test_user_data

In test_user_data, there is duplicated code for creating a request.
In addition, there is an internal method which works as almost same
as the duplicated code.
This patch makes the tests use the internal method and removes the
duplicated code for cleanup.

Change-Id: Ie87cc14a3ac67bc1262de155dc0a1dcf5c5e7d9a
Merge "More informative nova-scheduler log after NoValidHost is caught."
Merge "Remove duplicated code in test_user_data"