[BUG] Parameter `total` of function `asPercent()` has a wrong type defined
**Describe the bug**
Under certain circumstances the parameter `total` of the function `asPercent()` may have different types, depending on other specified parameters. But the type assigned with this parameter is always `seriesList`, this isn't always correct: https://github.com/graphite-project/graphite-web/blob/master/webapp/graphite/render/functions.py#L871
This may cause the input validation to incorrectly consider a the given value invalid

**To Reproduce**
Enforce input validation and use the `asPercent()` function with a numeric value as `total`

**Expected behavior**
This should be considered valid

**Additional context**
I'll make a PR to fix it

diff --git a/webapp/graphite/functions/params.py b/webapp/graphite/functions/params.py
index 3c5398d9..ea730e27 100644
--- a/webapp/graphite/functions/params.py
+++ b/webapp/graphite/functions/params.py
@@ -72,6 +72,9 @@ ParamType.register('seriesLists', validateSeriesList)
 ParamType.register('string')
 ParamType.register('tag')
 
+# special type that accepts everything
+ParamType.register('any')
+
 
 class ParamTypeAggFunc(ParamType):
 
diff --git a/webapp/graphite/render/functions.py b/webapp/graphite/render/functions.py
index 2dcbc85f..0fcbbcce 100644
--- a/webapp/graphite/render/functions.py
+++ b/webapp/graphite/render/functions.py
@@ -868,7 +868,7 @@ def asPercent(requestContext, seriesList, total=None, *nodes):
 asPercent.group = 'Combine'
 asPercent.params = [
   Param('seriesList', ParamTypes.seriesList, required=True),
-  Param('total', ParamTypes.seriesList),
+  Param('total', ParamTypes.any),
   Param('nodes', ParamTypes.nodeOrTag, multiple=True),
 ]
 
diff --git a/webapp/tests/test_functions.py b/webapp/tests/test_functions.py
index 8eace86a..0010dfd5 100644
--- a/webapp/tests/test_functions.py
+++ b/webapp/tests/test_functions.py
@@ -6176,7 +6176,7 @@ class FunctionsTest(TestCase):
                 },
                 {
                     'name': 'total',
-                    'type': 'seriesList'
+                    'type': 'any'
                 },
                 {
                     'multiple': True,
['webapp/graphite/render/functions.py', 'webapp/tests/test_functions.py', 'webapp/graphite/functions/params.py']