qa/tasks/qemu: open file in text mode

qa/tasks/qemu: open file in text mode

in python3, if a file is opened in 'rb' mode, we will have bytes when
reading from it, as a result, we have following failure like:
```
Traceback (most recent call last):
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/contextutil.py", line 32, in nested
    vars.append(enter())
  File "/usr/lib/python3.6/contextlib.py", line 81, in __enter__
    return next(self.gen)
  File "/home/teuthworker/src/github.com_tchaikov_ceph_nautilus-py3/qa/tasks/qemu.py", line 124, in generate_iso
    test_setup = ''.join(f.readlines())
TypeError: sequence item 0: expected str instance, bytes found
```

Signed-off-by: Kefu Chai <kchai@redhat.com>