cli: parametrisable run-example command

Allows to run any example in the `run-example` command.

Moves prefetching and loading of example environment images from the
cluster creation step to later stages.

Allows to speed up local testing using (i) one small fast REANA example
in the CI cycle vs (ii) using all examples for a full-scale CI testing.

Signed-off-by: Tibor Šimko <tibor.simko@cern.ch>

Author: tiborsimko

reana/cli.py

@@ -1879,8 +1879,8 @@ def client_setup_environment(server_hostname, insecure_url):  # noqa: D301
     "--component",
     "-c",
     multiple=True,
-    default=["reana-demo-root6-roofit"],
-    help="Which examples to run? [reana-demo-root6-roofit]",
+    default=["DEMO"],
+    help="Which examples to run? [default=DEMO]",
 )
 @click.option(
     "--workflow_engine",
@@ -1922,6 +1922,10 @@ def run_example(
 ):  # noqa: D301
     """Run given REANA example with given workflow engine.
 
+    \b
+    Example:
+       $ reana-dev run-example -c r-d-r-roofit
+
     \b
     :param component: The option ``component`` can be repeated. The value is
                       the repository name of the example. The special value `DEMO`
@@ -2033,7 +2037,7 @@ def run_example(
     default=False,
     help="Should we mount the REANA source code for live code updates?",
 )
-@click.option("--worker-nodes", default=0, help="How many worker nodes? default 0")
+@click.option("--worker-nodes", default=0, help="How many worker nodes? [default=0]")
 @cli.command(name="cluster-create")
 def cluster_create(mounts, debug, worker_nodes):
     """Create cluster."""
@@ -2088,8 +2092,8 @@ def add_volume_mounts(node):
     cluster_create = cluster_create.format(cluster_config=yaml.dump(cluster_config))
     for cmd in [
         cluster_create,
-        "reana-dev docker-pull -c reana -c DEMO",
-        "reana-dev kind-load-docker-image -c reana -c DEMO",
+        "reana-dev docker-pull -c reana",
+        "reana-dev kind-load-docker-image -c reana",
     ]:
         run_command(cmd, "reana")