Bug 1159067 - [RFE] 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.
Setting username to None on line 210 is harmless, but it is guaranteed to be set by at least this line so it's not necessary. I'm fine if you want to leave it though.