Use string.ascii_letters to be python 3 compatible

Use string.ascii_letters instead of string.letter. The only consequence
is to loose the locale-specific behavior but it seems not to be a bad
thing for generating a secret key.

Change-Id: Id985d798042eaa7541692d137027233537ed3bb4
Closes-Bug: #1360861
Merge "Use string.ascii_letters to be python 3 compatible"