ceph-detect-init: fix the py3 test

ceph-detect-init: fix the py3 test

the mock_open() does not support "for i in open()", and readline() and
readlines() support was added in py3.3 and py3.4 so for better backwards
compatibility, we should change the code being tested to use the plain
read() call.
and also use open(path) instead of open(path, 'rb') for simplicity,
otherwise we need to use bytestring for comparison and pass the same
parameters to mock.

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