Increase the timeout for charmstore connections (#262)

* Increase the timeout for charmstore connections

The charm store often does not respond in 5s. Increase the timeout
to 20s. Charm store issue being tracked in canonical/jujucharms.com#551
. Closes issue 261.

* Allow charm store timeout to be configurable

juju/model.py

@@ -2160,9 +2160,9 @@ class CharmStore:
     """
     Async wrapper around theblues.charmstore.CharmStore
     """
-    def __init__(self, loop):
+    def __init__(self, loop, cs_timeout=20):
         self.loop = loop
-        self._cs = theblues.charmstore.CharmStore(timeout=5)
+        self._cs = theblues.charmstore.CharmStore(timeout=cs_timeout)
 
     def __getattr__(self, name):
         """