Support pudb as a different post mortem debugger

Post mortem debugger starts pdb at the assert failure place
when OS_POST_MORTEM_DEBUGGER is set and an exception or assert
failure happens.

This patch adds a neutron env variable OS_POST_MORTEM_DEBUGGER
to allow invocation of pudb instead of pdb. pudb module is not
a hard requisite as it will be only imported if you set
OS_POST_MORTEM_DEBUGGER=pudb.

The old OS_POST_MORTEM_DEBUG env variable is removed now.

Change-Id: I5d40913add439cf9c30305bafc98af9f8cd4d12f
Merge "Support pudb as a different post mortem debugger"