Introduce runners

We've been using a generic `osbuild-run`, which sets up the build
environment (and works around bugs) for all build roots. It is already
getting unwieldy, because it tries to detect the OS for some things it
configures. It's also about to cause problems for RHEL, which doesn't
currently support a python3 shebang without having /etc around.

This patch changes the `build` key in a pipeline to not be a pipeline
itself, but an object with `runner` and `pipeline` keys. `pipeline` is
the build pipeline, as before. `runner` is the name of the runner to
use. Runners are programs in the `runners` subdirectory.

Three runners are included in this patch. They're copies of osbuild-run
for now (except some additions for rhel82). The idea is that each of
them only contains the minimal setup code necessary for an OS, and that
we can review what's needed when updating a build root.

Also modify the `--build-pipeline` command line switch to accept such a
build object (instead of a pipeline) and rename it accordingly, to
`--build-env`.

Correspondingly, `OSBUILD_TEST_BUILD_PIPELINE` → `OSBUILD_TEST_BUILD_ENV`.

README.md

@@ -71,24 +71,25 @@ The above pipeline has no base and produces a qcow2 image.
 ## Running
 
 ```
-usage: python3 -m osbuild [-h] [--build-pipeline PIPELINE] [--store DIRECTORY]
-                   [-l DIRECTORY]
+usage: __main__.py [-h] [--build-env ENV] [--store DIRECTORY] [-l DIRECTORY]
+                   [--json]
                    PIPELINE
 
 Build operating system images
 
 positional arguments:
-  PIPELINE              json file containing the pipeline that should be built
+  PIPELINE              json file containing the pipeline that should be
+                        built, or a '-' to read from stdin
 
 optional arguments:
   -h, --help            show this help message and exit
-  --build-pipeline PIPELINE
-                        json file containing the pipeline to create a build
+  --build-env ENV       json file containing a description of the build
                         environment
   --store DIRECTORY     the directory where intermediary os trees are stored
   -l DIRECTORY, --libdir DIRECTORY
                         the directory containing stages, assemblers, and the
                         osbuild library
+  --json                output results in JSON format
 ```
 
 ### Running example