ms: Make the outer variables configurable

Do the easy part for making these variables configurable.

Change-Id: If8bbedcf672f6481a12c30d3669564704063626c
diff --git a/src/osmo_ms_driver/cdf.py b/src/osmo_ms_driver/cdf.py
index 781349b..e163cc3 100644
--- a/src/osmo_ms_driver/cdf.py
+++ b/src/osmo_ms_driver/cdf.py
@@ -103,3 +103,10 @@
     scale = 1.0/duration.total_seconds()
     return DistributionFunctionHandler(step_size, duration,
                                         lambda x: _in_out(x*scale))
+
+
+cdfs = {
+    'immediate': lambda x,y: immediate(y),
+    'linear': linear_with_duration,
+    'ease_in_out': ease_in_out_duration,
+}