update the kepler internal e2e to accept kepler image as env

[vprashar2929]

Currently, in the case of Kepler internal e2e, it takes Kepler image as a const. It would be nice to take Kepler's image as env. If env is not set then it should fall back to what's there in const.
Ref:

kepler-operator/tests/e2e/kepler_internal_test.go

Line 37 in bd8ac42

const (