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.
"The priority list for pulp-admin to use the user credentials is as follows" -> "pulp-admin finds username and password credentials in the following order:"