Replace assert_ with assertTrue

The method assert_ has been deprecated since python 2.7.
http://docs.python.org/2/library/unittest.html#deprecated-aliases

This patch is a mere sed replace of one-liner assert_ to assertTrue.

grep -lR "assert_(" nova/tests/* | \
xargs sed -ri 's/(\W+self.assert)_(\()(.+)(\)$)/\1True\2\3\4/'

Change-Id: I64b9764e1607a46877dad466a4be81e07bdf6f9a