Substitute Jinja variables in Dovecot configuration

Summary:
This is a follow-up for f6419f2b78d8.

Ref T1931.

Test Plan:
Deployed on Hervil, 10-mail.conf refers correct path:
mail_location = maildir:/var/mail/_virtual

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T1931

Differential Revision: https://devcentral.nasqueron.org/D3491