Remove hardcoded config file in error message
Remove hardcoded config file in error message

Previously, it throw error message when database connection was
not defined,

"ValueError: Database connection not set in /etc/magnum/magnum.conf"

even if it was explicitly run with "--config-file /other/location".
So remove that hardcoded config file location.

Change-Id: I3af00f1f6572c3cd9e56141e1774acf373bbce8b
Closes-Bug: #1487258

Alzon/SUR,eshijia/magnum,openstack/magnum,dimtruck/magnum,Tennyson53/SUR,ArchiFleKs/magnum,eshijia/SUR,mjbrewer/testindex,jay-lau/magnum,Tennyson53/SUR,ddepaoli3/magnum,Tennyson53/magnum,eshijia/magnum,eshijia/SUR,annegentle/magnum,annegentle/magnum,ddepaoli3/magnum,ArchiFleKs/magnum,mjbrewer/testindex,openstack/magnum,Tennyson53/magnum,mjbrewer/testindex,Alzon/SUR,dimtruck/magnum