Fix (python): use of braces in format strings

Using braces in format strings without specifying a number (e.g. "The
number {} is incorrect".format(some_variable)) is available since Python
2.7, and older Python 2.6 required a number, zero-based (e.g. "The
number {0} is incorrect").

This patch adds these numbers for the sake of older systems.

Acked-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Signed-off-by: Amit Margalit <amitm@il.ibm.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>