do not join to_addr, smtplib expects a list

When configuring multiple addresses to send email via reporting function at the end of a deploy, cobbler was joining the list of email addresses with ", ".  Unfortunately this configuration results in the message not being properly parsed by smtplib as it expects a list.  With the join messages sent to multiple recipients were received by a single recipient, but without the message is received by all.

Removing the join allows mail to be sent to multiple recipients as expected when configured with:

build_reporting_email: [ 'a@mydomain.com', 'b@mydomain.com', 'c@mydomain.com' ]