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.
I think removing this test case is reasonable. I'm not convinced this code needs tests since it just reads a value from a config file, but here are three tests that could be written. It's up to you if you want to write these since the new behavior introduced is so small.

* assert that the conf file is read only if the user does not specify both username and password
* assert the default username is 'admin'
* assert the default password is 'admin'