Restart IMAP server too when deploying new mail SSL certificate

Summary:
Trigger a restart dovecot service after postfix.

Fixes T1110. Ref T919.

This is a follow-up for change 4d9fcc57d6fc.

Test Plan:
Tested on Dwellers to propagate certificate,
server has been successfully restarted.

Reviewers: xcombelle, dereckson

Reviewed By: dereckson

Subscribers: Sandlayth

Maniphest Tasks: T1110, T919

Differential Revision: https://devcentral.nasqueron.org/D836