Require use_sell_signal to be true for edge

Otherwise edge will have strange results, as
edge runs with sell signal, while the bot runs without sell signal,
causing results to be invalid

closes #3900

tests/test_configuration.py

@@ -812,6 +812,21 @@ def test_validate_edge(edge_conf):
     validate_config_consistency(edge_conf)
 
 
+def test_validate_edge2(edge_conf):
+    edge_conf.update({"ask_strategy": {
+        "use_sell_signal": True,
+    }})
+    # Passes test
+    validate_config_consistency(edge_conf)
+
+    edge_conf.update({"ask_strategy": {
+        "use_sell_signal": False,
+    }})
+    with pytest.raises(OperationalException, match="Edge requires `use_sell_signal` to be True, "
+                       "otherwise no sells will happen."):
+        validate_config_consistency(edge_conf)
+
+
 def test_validate_whitelist(default_conf):
     default_conf['runmode'] = RunMode.DRY_RUN
     # Test regular case - has whitelist and uses StaticPairlist