Map emails to usernames using a configuration file.

Add user configuration file to define email addresses for usernames.
Use the file to get usernames for Jira from email addresses on
PagerDuty. Move the methods to generate descriptions and comments
of issues to the PagerDutyToJira class to be able to use the user
configuration.
Use hostname only to search for issue.

For incidents triggered by Nagios, only the hostname used to search
for the last updated issue. It would cause to open less duplicated
issues on our usecase. Configuration parameters can be added later
to make it more generalized.

Remove the optimization to avoid searching when issue key exists
on the subject field of the summary as it can only be used on trigger
action.