Fixed open tempfile.NamedTemporaryFile as text in Python3
Fixed open tempfile.NamedTemporaryFile as text in Python3

Python3 opens tempfile by default as binary[1] and not as ascii,
and nvmetcli writes json output as ascii.

This issue was overlooked with Python2, since binary and ascii
variables are less distiguished there.

[1] https://docs.python.org/3/library/tempfile.html
[2] https://bugs.python.org/issue29245

Change-Id: Ia1f280ab64f0dc90824d4cb9ed20a8537af32ef6
Closes-Bug: 1855855

openstack/cinder,openstack/cinder,mahak/cinder,mahak/cinder