Since https://review.opendev.org/647699/, we lost the logic to only
deploy glance-api on a single host when using the file backend.

This code was always a bit custom, and would be better supported by
using the 'host_in_groups' pattern we have in a few other places where a
single group name does not describe the placement of containers for a
service.

Change-Id: I21ce4a3b0beee0009ac69fecd0ce24efebaf158d
Closes-Bug: #1836151