Set issue_regexp from env variable.

[jozsef-armin-hamos]

Hello. We have lots of repos where we want to implement the jira notifications, and wanted to set in a central CircleCI context, the env variable for a custom regexp. Unfortunately setting the JIRA_VAL_ISSUE_REGEXP env variable doesn't have any effect, and the default regexp is only changed if the issues_regexp parameter is used.
Can we have the option to use an env variable for setting a custom regexp instead of using the parameter everywhere?