feat: add ability to check for multiple default ssh key locations

Since Ubuntu 24.04 LTS and newer, the default location for ssh keys is
`~/.ssh/id_ed25519` instead of `~/.ssh/id_rsa`. Thus, when no ssh key
path is specified, this commit changes pycloudlib to check for an
existing ssh key at either  `~/.ssh/id_ed25519` or `~/.ssh/id_rsa`.

Fixes GH canonical#341

pycloudlib/cloud.py

@@ -62,14 +62,26 @@ def __init__(
         self._check_and_set_config(config_file, required_values)
 
         user = getpass.getuser()
+        # check if id_rsa or id_ed25519 keys exist in the user's .ssh directory
+        possible__default_keys = [
+            os.path.expandvars(f"~{user}/.ssh/id_rsa.pub"),
+            os.path.expandvars(f"~{user}/.ssh/id_ed25519.pub"),
+        ]
+        for target_public_key in possible__default_keys:
+            if os.path.exists(target_public_key):
+                break
+        public_key_path = os.path.expandvars(
+            self.config.get(
+                "public_key_path",
+                target_public_key,
+            )
+        )
+        self._log.debug("using SSH pubkey: %s", public_key_path)
+        private_key_path = self.config.get("private_key_path", "")
         self.key_pair = KeyPair(
-            public_key_path=os.path.expandvars(
-                self.config.get("public_key_path", f"~{user}/.ssh/id_rsa.pub")
-            ),
-            private_key_path=os.path.expandvars(
-                self.config.get("private_key_path", "")
-            ),
-            name=self.config.get("key_name", user),
+            public_key_path,
+            private_key_path,
+            self.config.get("key_name", user),
         )
 
         self.tag = get_timestamped_tag(tag) if timestamp_suffix else tag