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.
Note, the admin.conf from above is the systemwide admin.conf, but this snippet is from a file that the user needs to create on their own. We talk about creating this file elsewhere in the docs, so I think this snippet needs to recommend the user add this section. I recommend rewriting this to:

"# The following snippet is from ~/.pulp/admin.conf" -> "# Add the following snippet to ``~/.pulp/admin.conf``"