Import packages_prefixes in ViMbAdmin states

Summary:
This change fixes the following issue:

hervil:

Data failed to compile:

Rendering SLS 'base:roles/mailserver/vimbadmin' failed: Jinja variable 'packages_prefixes' is undefined; line 28

Fixes T1977.

Test Plan: salt hervil state.show_sls roles/mailserver/vimbadmin

Reviewers: DorianWinty, dereckson

Reviewed By: dereckson

Maniphest Tasks: T1977

Differential Revision: https://devcentral.nasqueron.org/D3543