Merge branch 'patch-telegraf' into 'master'

Update main.yml to bump telegraf from 1.12 to 1.15

See merge request !357

roles/telegraf/vars/main.yml

-telegraf_install_rpm_url: https://dl.influxdata.com/telegraf/releases/telegraf-1.12.6-1.x86_64.rpm
-telegraf_install_deb_url: https://dl.influxdata.com/telegraf/releases/telegraf_1.12.6-1_amd64.deb
+telegraf_install_rpm_url: https://dl.influxdata.com/telegraf/releases/telegraf-1.15.2-1.x86_64.rpm
+telegraf_install_deb_url: https://dl.influxdata.com/telegraf/releases/telegraf_1.15.2-1_amd64.deb