Increase timeout per upgrade path to 15mins

From recently added logs it seems that the longest upgrade path,
(starting with `4.0.x`), frequently consumes a bit more than 10 mins.

Author: matriv

tests/bwc/test_upgrade.py

@@ -209,7 +209,7 @@ def _test_upgrade_path(self, versions: Tuple[VersionDef, ...], nodes: int):
                     f.truncate()
                     f.close()
 
-    @timeout(600)
+    @timeout(900)
     def _do_upgrade(self,
                     cluster: CrateCluster,
                     nodes: int,