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.
"credentials stored in user's ``~/.pulp/admin.conf`` is second priority" -> "credentials set in user's ``~/.pulp/admin.conf``"