Added allow_larger_keys flag to custom policies to control whether targets can have larger keys, and added Docker tests to complete work started in PR #242.

test/docker/expected_results/openssh_8.0p1_custom_policy_test17.json

@@ -0,0 +1,6 @@
+{
+    "errors": [],
+    "host": "localhost",
+    "passed": true,
+    "policy": "Docker policy: test17 (version 1)"
+}