support mixed parameters referencing / not referencing other stacks

Currently parameters can reference multiple outputs in other stacks:
```
SomeParameter: stack1::Output1,stack2::Output2
```

This commit allows parameter lists to also include variables not
referencing another stack, e.g.:
```
SomeParameter: ${ThingInEnvFile},stack1::Output1
```

Behind the scenese, now values will always split on commas,
eliminating one of the nested conditionals.

Also changed 2 of the `if`s to `elif`s; probably won't make any
performance impact, but since each condition is independent it feels
more precise to me.

stacker/actions/build.py

@@ -83,11 +83,16 @@ def resolve_parameters(parameters, blueprint, context, provider):
                          blueprint.name, k)
             continue
         value = v
-        if isinstance(value, basestring) and "::" in value:
-            # Get from the Output(s) of another stack(s) in the stack_map
-            v_list = []
+        # v_list holds each value in the (potentially) comma-delimited list of
+        # values for each parameter
+        v_list = []
+        if isinstance(value, basestring):
             values = value.split(",")
-            for v in values:
+        else:
+            values = [value]
+        for v in values:
+            if isinstance(v, basestring) and "::" in v:
+                # Get from the Output(s) of another stack(s) in the stack_map
                 v = v.strip()
                 stack_name, output = v.split("::")
                 stack_fqn = context.get_fqn(stack_name)
@@ -96,17 +101,19 @@ def resolve_parameters(parameters, blueprint, context, provider):
                         provider.get_output(stack_fqn, output))
                 except KeyError:
                     raise exceptions.OutputDoesNotExist(stack_fqn, v)
-            value = ",".join(v_list)
-        if value is None:
-            logger.debug("Got None value for parameter %s, not submitting it "
-                         "to cloudformation, default value should be used.",
-                         k)
-            continue
-        if isinstance(value, bool):
-            logger.debug("Converting parameter %s boolean \"%s\" to string.",
-                         k, value)
-            value = str(value).lower()
-        params[k] = value
+            elif value is None:
+                logger.debug("Got None value for parameter %s, not submitting "
+                             "it to cloudformation, default value should be "
+                             "used.", k)
+                continue
+            elif isinstance(value, bool):
+                logger.debug("Converting parameter %s boolean \"%s\" to "
+                             "string.", k, value)
+                v_list.append(str(value).lower())
+            else:
+                v_list.append(value)
+        if v_list != []:
+            params[k] = ",".join(v_list)
     return params
 
 

stacker/stack.py

@@ -88,6 +88,9 @@ def requires(self):
                 # support for list of Outputs
                 values = value.split(",")
                 for x in values:
+                    # Skip parameters not referencing another stack
+                    if '::' not in x:
+                        continue
                     stack_name, _ = x.split("::")
                     stack_names.append(stack_name)
             else: