In Python 2.6 it was required to use {0}, {1}...{n} when using the
string format function. In Python 2.7 and Python 3 it it not required.
Change {N} to {} in code.
This brings the code in style alignment with other projects like
ironic and ironic-lib.
Change-Id: I81c4bb67b0974f73905f14b589b3dd0a7131650d
Depends-On: I8f0e5405f3e2d6e35418c73f610ac6b779dd75e5