The support for specifying multiple `image_types` for a single compose
has been removed by [1]. This turned out to be problematic, because e.g.
Pungi uses the array type when triggering image builds via osbuild.
Bring back the support for specifying the `image_type` as an array, but
restrict it to a single item. This will cover the Pungi use-case, since
it is always passing a single `image_type` in the array. The array is
then converted to a string in the Hub plugin and passed as such to the
Builder plugin.
Extend unit tests covering the introduced compatibility layer.
[1] c725265081
While it is technically possible to build more than one image type
as part of a Koji compose in osbuild-composer, this option is not used
in reality and it also makes very little sense. If the user wants to
build more than one image type, they should submit multiple Koji builds.
Adjust affected unit tests.
Add a new command line option `--skip-tag` that will skip tagging
after a successful build. The help text is copied from the same
option of other sub-commands in the koji client. The hub plugin's
jsonschema was updated accordingly, and the builder plugin will
skip the tag if the option was requested.
Tests were added or augmented for all three plugins to test the
new option.
Create a new plugintest.PluginTest class that shares the plugin
loading code that is common to all plugin testing. Adapt the
existing (hub, builder) tests.
Also correct the name for test_builder.TestHubPlugin to
TestBuilderPlugin.
