Prevent S3 information on non-S3 mirrors

Switch from looking at the presence of the S3 information keys to
determine if the dictionary URL is used to instead look at the value
in the keys.

Add s3_endpoint_url as an additional key value for the S3 information.

lib/spack/spack/mirror.py

@@ -528,9 +528,10 @@ def add(name, url, scope, args={}):
 
     items = [(n, u) for n, u in mirrors.items()]
     mirror_data = url
-    key_values = ["s3_access_key_id", "s3_access_token", "s3_profile"]
+    key_values = ["s3_access_key_id", "s3_access_token", "s3_profile", "s3_endpoint_url"]
     # On creation, assume connection data is set for both
-    if any(value for value in key_values if value in args):
+    # Check for value in each key, instead of presence of each key
+    if any(vars(args)[value] for value in key_values if value in args):
         url_dict = {"url": url,
                     "access_pair": (args.s3_access_key_id, args.s3_access_key_secret),
                     "access_token": args.s3_access_token,