[dock] Allow to disable discovery of environments

Allows to speed up manipulation if the environment is already
known and available.

beat/core/dock.py

@@ -55,7 +55,7 @@ class Host(object):
     images_cache = {}
 
 
-    def __init__(self, images_cache=None, raise_on_errors=True):
+    def __init__(self, images_cache=None, raise_on_errors=True, discover=True):
 
         # Initialisations
         self.raise_on_errors = raise_on_errors
@@ -71,7 +71,8 @@ class Host(object):
                 Host.images_cache = simplejson.load(f)
 
         # Discover the environments
-        (self.processing_environments, self.db_environments) = self._discover_environments()
+        if discover:
+            (self.processing_environments, self.db_environments) = self._discover_environments()
 
         # (If necessary) Save the known infos about the images
         if self.images_cache_filename is not None: