By unknown reasons(at least for me), output of `nova update` command for
description field started fail.
Test name:
TestServersDescription.test_update_with_description_longer_than_255_symbols
The error message doesn't include new line at the start of message anymore.
Also, I don't know why new line was there in the past. The main behavior and
error message are not changed, so let's remove '\n' from assertation and
unblock gates.
Change-Id: I8442fe837985600250ccdf0d1398424f1fca2185