check for existence of output env file for container image prepare
check for existence of output env file for container image prepare

Currently, `overcloud container image prepare --env-file` will clobber
any existing file, which can present a problem.

This patch catches this scenario, moves the existing --env-file to a
backup, and then continues with the creation.

Change-Id: I3d88ab7a948a1e14940542964824a3bbc1a94fa6
Closes-Bug: 1723071

openstack/python-tripleoclient