Fix triggering without a pagerduty user

https://github.com/hubot-scripts/hubot-pager-me/pull/12 introduced a regression where `/pager trigger` required the user to have a PagerDuty account. 

Previously, the script required a service to trigger against, assuming there was an single escalation policy assigned to it. Now that we support multiple schedules/escalation policies, we moved to a single service that wasn't assigned to an escalation policy (technically, assigned to a escalation policy with a no-op user). The incident gets triggered, and then reassigned to the correct user or escalation policy. The reassignment requires a `requester_id` parameter, so PagerDuty can know who initiated the act. That is why it's requiring a user.

This pull request addresses this by adding an environment variable to set a default  user id to use for requester_id when reassigning the incident.