Fix undefined variables in Munin template

In development, allow for certain variables to be undefined,
because emails and Slack notifications don't have to be sent.

- Move location of email variable and allow for munin_master
  environment-specific variable files.

- Include vars files in munin_master play.

- Remove email variable from example files
Merge pull request #38 from dpla/7747-fix-munin-template

Fix undefined variables in Munin template