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 don't think this function is needed. The Config object in L#54 reads the config file but if values are not present it uses the defaults you put in L#45 - L#48 in client_admin/pulp/client/admin/config.py. This will never cause a KeyError to happen so the branch on L#216 will never occur. Overall it would be simplest if ``username = config['auth']['username']`` was used on L#67 and this function was removed.