pybind/mgr/restful: use list to pass hooks to create a `Pecan` instance

pybind/mgr/restful: use list to pass hooks to create a `Pecan` instance

pecan 0.3.2 introduced the new way to pass the hooks to construct a
Pecan instance, but the "callable" builder for hooks is not compatible
with pecan < 0.3.2. and ubuntu trusty ships pecan 0.3.0, so we need to
do this the old way: pass a list of hooks instead of a callable.

see https://github.com/pecan/pecan/blame/0.3.2/pecan/core.py and
https://github.com/pecan/pecan/commit/fb2cf166aaf5b5af461879ec5b7a46806e8aee45

Fixes: http://tracker.ceph.com/issues/20258
Signed-off-by: Kefu Chai <kchai@redhat.com>