emails - add default From to login/password emails

It was observed that with the nightly build the password reset
and login emails were no longer being sent for RHEL server
configurations.

During investigation, found the following in the delayed_jobs.log:

2013-01-10T12:10:41-0500: [Worker(delayed_job host:katello.com pid:28884)]
Class#logins failed with ArgumentError: A sender (Return-Path, Sender or From)
required to send a message - 0 failed attempts

In order to address this, we are adding a configuration property
(email_reply_address) to the katello.yml that may be used to specify
the address. If not specified, a default of 'no-reply@host' will be
used, where host is the value specified for that property.

Note: in the future, we may want to add this to the UI in the
form of a database attribute; however, we might want to consider
having the email specified at different levels possibly based on
org or classification of email (e.g. login related emails, consumer
related emails...etc)