Allow file engine to be used with other engines

[tgerdes]

A cloud should be allowed to enable file and rbd or swift backends.
This check prevented other backends from being enabled if file was in
the list.

[pupapaik]

I disagree, because in the past I needed to have file,http to run it properly.

[tgerdes]

How do you suggest me enabling

storage_engines: rbd,file,http

In my cloud then?

This bug prevents me from doing that, the presence of file in my pillar removes rbd from the generated config

[tgerdes]

Perhaps there confusion here about backwards compatibility. This patch preserves the previous behavior while making the option more flexible.

In the template, storage_engines is a python list of the pillar data server.storage.engine split on commas. (see https://github.com/tgerdes/salt-formula-glance/blob/f8b4128c275c7c5a7cf77cb315becd228d70f136/glance/files/mitaka/glance-api.conf.Debian#L2 )

With this patch, If a user specifies a pillar of:

glance:
    server:
        storage:
            engine: file

The generated template will be

default_store = file
stores = file,http

A pillar of

glance:
    server:
        storage:
            engine: "rbd,file,http"

Generates

default_store = rbd
stores = rbd,file,http

With this new scheme, a user that is using a comma separated string to specify multiple engines is expected to include all required engines (in the case where "file" implies "http" should be enabled).

This patch does not accommodate any way to generate a config that sets

default_store = file
stores = file

Before this patch, the pillar

A pillar of

glance:
    server:
        storage:
            engine: "rbd,swift,file,http"

would generate the config:

default_store = file
stores = file,http

Which is missing the user requested setting.

[tgerdes]

Adding @samos123 for additional review.