Bug 1159067 - Read user cred from config

Bug 1159067 - [RFE] allow pulp to read user credentials from users admin.conf
if he/she does not want to provide password on command prompt
or --password in pulp-admin

this is basically for writing a batch job that would include multiple command lines together.
After thinking more about this, I think this note needs to be rewritten to be clearer on security. Here's my suggestion:
```
# This enables all system users to run pulp-admin commands using the username
# and password specified here.
# 
# WARNING: This file is world-readable so only use a password here that is
#          appropriate for any system user to read.
#
```