doc: How to pdb under systemd
doc: How to pdb under systemd

Add a section to the systemd doc describing how to use regular pdb in a
systemd environment.

Change-Id: Ib1f616f407eccc087de1c60624fa74e2555971c2

group-policy/devstack,openstack-dev/devstack,nawawi/openstack,nawawi/openstack,noironetworks/devstack,noironetworks/devstack,dtroyer/devstack,dtroyer/devstack,group-policy/devstack,openstack-dev/devstack