[bug 581] Fix parameter checking if no data-type

Fix bug #581, which reported that the deployment of the simplecharm
failed because the filename parameter was invalid.

This turned out to be an issue introduced with the parameter
type-checking added for R5. This has been fixed, and an integration test
has been added that exercises the simplecharm example.

Signed-off-by: Adam Israel <adam.israel@canonical.com>
diff --git a/tests/charms/layers/simple/layer.yaml b/tests/charms/layers/simple/layer.yaml
new file mode 100644
index 0000000..3fed5e2
--- /dev/null
+++ b/tests/charms/layers/simple/layer.yaml
@@ -0,0 +1,4 @@
+includes: ['layer:basic', 'layer:vnfproxy']
+options:
+        basic:
+                use_venv: false