pybind/ceph_daemon: do not fail if prettytable is not available

pybind/ceph_daemon: do not fail if prettytable is not available

ubuntu focal does not package python-prettytable. but we need to run
"make check" on focal. it turns out the prettytable is not a must have
for running "make check", so just skip it if it is not around.

this change is not cherry-picked from master, as we have dropped python2
support in master, and python3-prettytable is packged fro python3 on
ubuntu focal. also nautilus is the latest release which has python2
support.

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