qa/tasks/cephfs/mount.py: try to flush the stale ceph-brx dev info

qa/tasks/cephfs/mount.py: try to flush the stale ceph-brx dev info

Once we have run the test cases and the ceph-brx bridge is setup,
it will save the config in "/etc/sysconfig/network-scripts/ifcfg-ceph-brx"
or somewhere else. It will be kept after the ceph-brx bridge removed.
So next time once the ceph-brx bridge is created or added, it will
read the config from it, then when we config it again we will get
error like:

    "RTNETLINK answers: File exists"

Here we need to flush it before config it.

Fixes: https://tracker.ceph.com/issues/45817
Signed-off-by: Xiubo Li <xiubli@redhat.com>