Give a default to sample-program (Qiskit#77)

* Give a default to sample-program

Seeing a lot of failed jobs from people just running the sample program without parameters. It kind of skews our metrics as noise when we look at job failures over time.

* Actually not require iterations param

* Update json schema

runtime/sample_program.json

@@ -12,12 +12,10 @@
         "iterations": {
           "type": "integer",
           "minimum": 0,
+          "default": 1,
           "description": "Number of iterations to run. Each iteration generates a runs a random circuit."
         }
-      },
-      "required": [
-        "iterations"
-      ]
+      }
     },
     "return_values": {
       "$schema": "https://json-schema.org/draft/2019-09/schema",

runtime/sample_program.py

@@ -40,9 +40,7 @@ def main(backend, user_messenger, **kwargs):
             program consumer.
         kwargs: User inputs.
     """
-    if "iterations" not in kwargs:
-        raise ValueError('"iterations" is a required input parameter.')
-    iterations = kwargs["iterations"]
+    iterations = kwargs.get("iterations", 1)
     for it in range(iterations):
         qc = prepare_circuits(backend)
         result = backend.run(qc).result()