Support specifying an identity file with GerritStatusPush

GerritChangeSource supports a knob for specifying a secret key to use
for connecting - make sure status push is symmetric.
Aha!  According to https://docs.python.org/2/library/string.html#format-examples, `{}` is ony supported starting 2.7.

Since minimal requirement for Buildbot is Python 2.6, this needs to be re-written in a supported way.