roles/cotainer_file_read: port to Python 3

Make it work on Python3, even if deprecated with the new `ssh_lxc`
connection plugin.

Now you can use directly the `slurp` native module with delegation to
the lxc guest.

May be useful only in case where it is impossible to get python3 or
other ansible requirements on the lxc guest.

library/container_file_read.py

@@ -30,7 +30,7 @@ def read_file_in_container(args):
                 path = path,
                 text = lines.read().strip('\n'),
             )
-    except IOError, e:
+    except IOError as e:
         module.exit_json(
             msg = e,
             path = path,