Resolve "Felix dies if interface missing" on Alpine

Alpine and Ubuntu have slightly different error messages when you
run 'ip link show <non-existent interface>'; respectively

    ip: can't find device 'thisdoesnotexist'
    Device "thisdoesnotexist" does not exist.

We run Felix inside an Alpine container for calico/node, but the
different wording meant Felix didn't recognise the missing
interface as a known error, so raised an exception.  Now we catch
both wordings and handle them correctly.

Fixes #899.

calico/felix/devices.py

@@ -91,7 +91,8 @@ def interface_exists(interface):
         futils.check_call(["ip", "link", "list", interface])
         return True
     except futils.FailedSystemCall as fsc:
-        if fsc.stderr.count("does not exist") != 0:
+        if ("does not exist" in fsc.stderr) or \
+           ("can't find device" in fsc.stderr):
             return False
         else:
             # An error other than does not exist; just pass on up