'databasefile' option for legacy storage backend

Fixes NixOS#1472.

nixops/storage/legacy.py

@@ -3,11 +3,12 @@
 import sys
 import os
 import os.path
+from typing import Optional
 from nixops.util import ImmutableValidatedObject
 
 
 class LegacyBackendOptions(ImmutableValidatedObject):
-    pass
+    databasefile: Optional[str]
 
 
 class LegacyBackend(StorageBackend[LegacyBackendOptions]):
@@ -18,7 +19,7 @@ def options(**kwargs) -> LegacyBackendOptions:
         return LegacyBackendOptions(**kwargs)
 
     def __init__(self, args: LegacyBackendOptions) -> None:
-        pass
+        self.args = args
 
     # fetchToFile: acquire a lock and download the state file to
     # the local disk. Note: no arguments will be passed over kwargs.
@@ -35,6 +36,9 @@ def state_location(self) -> str:
         if env_override is not None:
             return env_override
 
+        if self.args.databasefile is not None:
+            return self.args.databasefile
+
         home_dir = os.environ.get("HOME", "")
         charon_dir = f"{home_dir}/.charon"
         nixops_dir = f"{home_dir}/.nixops"