make string.letters python3 compatible
make string.letters python3 compatible

The string.letters in python2 was supported. But in python3,
string.letters was removed. In python3, recommend using
string.ascii_letters instead. This patch fix it.

Change-Id: Ib0444f1d527de4fcb0393bd04c93a64e9d79e3f9
Closes-Bug: #1595786

openstack/searchlight,openstack/searchlight,openstack/searchlight