T5249: Add rollback-soft feature

Add the ability to rollback configs without rebooting
```
sudo /usr/bin/config-mgmt rollback_soft --rev 1

rollback-soft 1
```

python/vyos/config_mgmt.py

@@ -261,6 +261,29 @@ def rollback(self, rev: int, no_prompt: bool=False) -> Tuple[str,int]:
 
         return msg, 0
 
+    def rollback_soft(self, rev: int):
+        """Rollback without reboot (rollback-soft)
+        """
+        msg = ''
+
+        if not self._check_revision_number(rev):
+            msg = f'Invalid revision number {rev}: must be 0 < rev < {maxrev}'
+            return msg, 1
+
+        path = os.path.join(archive_dir, f'config.boot.{rev}.gz')
+        with gzip.open(path, 'rb') as f:
+            config = f.read()
+
+        # Write config to laod
+        with open(rollback_config, 'wb') as f:
+            f.write(config)
+
+        rc, out = rc_cmd(f'/bin/cli-shell-api loadFile {rollback_config}')
+        if rc != 0:
+            raise ConfigMgmtError(out)
+
+        return msg, 0
+
     def compare(self, saved: bool=False, commands: bool=False,
                 rev1: Optional[int]=None,
                 rev2: Optional[int]=None) -> Tuple[str,int]:
@@ -703,6 +726,11 @@ def run():
     rollback.add_argument('-y', dest='no_prompt', action='store_true',
                           help="Excute without prompt")
 
+    rollback_soft = subparsers.add_parser('rollback_soft',
+                                     help="Rollback to earlier config")
+    rollback_soft.add_argument('--rev', type=int,
+                          help="Revision number for rollback")
+
     compare = subparsers.add_parser('compare',
                                     help="Compare config files")