Only install Slack notification scripts if Slack variables are defined
Merge pull request #37 from dpla/install_slack_script_check

Only install notify_slack when slack_webhook_url is defined
Fix undefined variables in Munin template

In development, certain variables don't have to be defined, because
emails and Slack notifications don't have to be sent.  Allow for this.

Also include the environment-specific config files in the top of the
task file.

See also e267a58f8f
Back out line that's part of another changeset

See https://github.com/dpla/automation/pull/37/files
Fix munin variable location

Add inclusion of vars files to munin_master play.

Move location of email variable and allow for munin_master
environment-specific variable files.
Fix variable file inclusion's path