docker: add "probe" command for configure

This is a helper function for the configure script. It replies yes,
sudo or no to inform the user if non-interactive docker support is
available. We trap the Exception to fail gracefully.
Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NFam Zheng <famz@redhat.com>

tests/docker/docker.py

@@ -390,6 +390,24 @@ class ImagesCommand(SubCommand):
     def run(self, args, argv):
         return Docker().command("images", argv, args.quiet)
 
+
+class ProbeCommand(SubCommand):
+    """Probe if we can run docker automatically"""
+    name = "probe"
+
+    def run(self, args, argv):
+        try:
+            docker = Docker()
+            if docker._command[0] == "docker":
+                print "yes"
+            elif docker._command[0] == "sudo":
+                print "sudo"
+        except Exception:
+            print "no"
+
+        return
+
+
 def main():
     parser = argparse.ArgumentParser(description="A Docker helper",
             usage="%s <subcommand> ..." % os.path.basename(sys.argv[0]))