supporting substitution of $FOO or ${FOO} invites confusion and makes it
hard to understand the behavior of a stackerfile by looking at it,
because it can accidentally hard-code a value that is expected to be a
shell variable in a run section.

using double brackets is visually different and has the benefit of being
invalid shell syntax, so you know that it is intended to be a stacker
placeholder.

Now if you specify --substitute FOO=bar and have $FOO or ${FOO} in your
stackerfile, stacker will fail the operation and print errors.

It first collects all instances of the error then prints them at the
end, so you shouldn't have to run it over and over to get them all.

Note that the previously documented behavior of allowing ${FOO:default}
was not tested and did not work. That documentation is changed to
reflect real life.

Test changes:

To avoid confusion about what substitution syntax we are testing, where
possible, use quoted heredocs for stackerfiles in tests.

This replaces use of e.g. $BUSYBOX_OCI, which was getting substituted by
the shell before cat wrote it to a file, with ${{BUSYBOX_OCI}} in a
quoted heredoc, so bash doesn't try to substitute it (which would fail),
and instead stacker will see it and substitute it itself.
This means adding a lot of --substitute args around where they were not
previously required.

A couple of places still use the previous approach so we can use the
shell to sub in a file sha, for example.

Signed-off-by: Michael McCracken <mikmccra@cisco.com>