fix string formatting when format contains a percent sign but no args

[ns-dfarino]

This patch fixes the case where the format string should be treated as a literal.

I was using dyff and saw this in my output:

rollingUpdate:
  maxSurge: 25%!(NOVERB)
  maxUnavailable: 25%!(NOVERB)

With this patch, I correctly see:

rollingUpdate:
  maxSurge: 25%
  maxUnavailable: 25%