ci: Add FCOS stream parameter

Most of the time, we want to build the module for the latest version in
the *testing* stream, so that it will be ready when that version moves
to *stable*.  There may be cases, though, where we need to build for a
different stream.  For example, if the module never built successfully
by the time a version moved from *testing* to *stable*, we may need to
manually build it for the current *stable*.

Jenkinsfile

@@ -1,7 +1,14 @@
 // vim: set sw=4 ts=4 sts=4 et :
 
 properties([
-    pipelineTriggers([cron('H H * * *')])
+    pipelineTriggers([cron('H H * * *')]),
+    parameters([
+        choice(
+            choices: ['testing', 'stable', 'next'],
+            description: 'Fedora CoreOS Stream',
+            name: 'stream',
+        )
+    ]),
 ])
 
 import groovy.json.JsonSlurper