lava_callback: Drop strict branch name checking in /checkout endpoint

Function "find_tree" verifies tree existence in the Pipeline config but
branch for "/checkout" request is taken from user's input.

This patch uses less strict check to allow checkouts from branches
absent in the static Pipeline config.

Signed-off-by: Paweł Wieczorek <pawiecz@collabora.com>

src/lava_callback.py

@@ -362,7 +362,7 @@ def find_tree(url, branch):
 
     for bconfig in YAMLCFG['build_configs']:
         data = YAMLCFG['build_configs'].get(bconfig)
-        if data.get('tree') == treename and data.get('branch') == branch:
+        if data.get('tree') == treename:
             return treename
 
     return None