change assert_ to assertTrue
change assert_ to assertTrue

According to http://docs.python.org/2/library/unittest.html
assert_ is a deprecated alias of assertTrue.

Change-Id: I18db0af72934dde4498502fb29579941d020cf01

pratikmallya/python-swiftclient,varunarya10/python-swiftclient,krnflake/python-hubicclient,VyacheslavHashov/python-swiftclient,VyacheslavHashov/python-swiftclient,jeseem/python-swiftclient,openstack/python-swiftclient,iostackproject/IO-Bandwidth-Differentiation-Client,iostackproject/IO-Bandwidth-Differentiation-Client,sohonetlabs/python-swiftclient,ironsmile/python-swiftclient,JioCloud/python-swiftclient,ironsmile/python-swiftclient,JioCloud/python-swiftclient,jeseem/python-swiftclient,varunarya10/python-swiftclient,sohonetlabs/python-swiftclient,pratikmallya/python-swiftclient,openstack/python-swiftclient