qa/tasks/cephadm: ignore tarfile.ReadError

qa/tasks/cephadm: ignore tarfile.ReadError

there is chance that /var/log/ceph (and other directories) is not
created when cephadm fails, but we always try to collect /var/log/ceph
by tar the directory on the test node and transfer it back for archiving
it. when we fail to do so, an exception is thrown, and this exception is
in turn logged as the cause of the failure instead of the one which
fails the test.

in this change, `tarfile.TarError`s is caught and ignored if it is
thrown by `misc.pull_directory()`, as normally, we don't consider it a fatal
failure when `misc.pull_directory()` fails because of `tar`. the
failures for the test are.

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