feat(CLI): allow specifying environment variables by name only (#234)

* feat(CLI): allow specifying environment variables by name only

* docs(CLI): update docs for `cluster vars new` and `cluster vars update`

Author: fubuloubu

silverback/_cli.py

@@ -926,7 +926,16 @@ def vars():
 
 def parse_envvars(ctx, name, value: list[str]) -> dict[str, str]:
     def parse_envar(item: str):
-        if not ("=" in item and len(item.split("=")) == 2):
+
+        if "=" not in item:
+            if not (envvar := os.environ.get(item)):
+                raise click.UsageError(
+                    f"Environment variable '{item}' has no value in your environment"
+                )
+
+            return item, envvar
+
+        elif len(item.split("=")) != 2:
             raise click.UsageError(f"Value '{item}' must be in form `NAME=VAL`")
 
         return item.split("=")
@@ -941,9 +950,9 @@ def parse_envar(item: str):
     "variables",
     multiple=True,
     type=str,
-    metavar="NAME=VAL",
+    metavar="NAME[=VAL]",
     callback=parse_envvars,
-    help="Environment variable key and value to add (Multiple allowed)",
+    help="Environment variable name or key and value to add (Multiple allowed)",
 )
 @click.argument("name")
 @cluster_client()
@@ -988,9 +997,9 @@ def vargroup_info(cluster: "ClusterClient", name: str):
     "updated_vars",
     multiple=True,
     type=str,
-    metavar="NAME=VAL",
+    metavar="NAME[=VAL]",
     callback=parse_envvars,
-    help="Environment variable key and value to add/update (Multiple allowed)",
+    help="Environment variable name or key and value to add/update (Multiple allowed)",
 )
 @click.option(
     "-d",