Replace pmap shellout with pure python implementation
Replace pmap shellout with pure python implementation

Without this patch, the pmap -XX call fails on openSUSE Leap
distributions as those have a fairly ancient procps version that
does not support the -XX parameter. A pure python implementation
is more portable, faster and even shorter than the subprocess
call.

Closes-Bug: #1716066
Change-Id: I2fdb457e65359a1c9d40452c922cfdca0e6e74dc

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