Change default alerts to empty hash
Change default alerts to empty hash

This should fix `promtool` parsing problems for prometheus 2.* and empty alert files. Behaviour for 1.* should remain unchanged.

Fixes #143
mdebruyn-trip/puppet-prometheus,dhoppe/puppet-prometheus,dhoppe/puppet-prometheus,dhoppe/puppet-prometheus,mdebruyn-trip/puppet-prometheus,mdebruyn-trip/puppet-prometheus