image build: separate the envvars for dib
image build: separate the envvars for dib

Put in a separate dictionary all the environment variables we need to
set, updating the current environment right before invoking
disk-image-create/ramdisk-image-create.

The result should be the same, but allowing to separately have the
changes to the environment for further processing.

Change-Id: I2867cd55231f84f83052d1342201dc9bcbe3dee6

openstack/python-tripleoclient