Fall back to $0 if BASH_SOURCE is unset

BASH_SOURCE does not exist in zsh or presumably many other shells.
$0 is good enough to locate the scripts directory and find the
os-set-type script, so use that instead.

Note this does not affect devtest.sh. That script always runs as bash,
because it has #!/bin/bash at the top. This fix is mostly useful if you
want to source devtest_variables.sh into your interactive shell. You
might want to do this to get os-apply-config on your path so you can
source overcloudrc.

Change-Id: I54456831a5c94a4c3ce76c606f31eec9b75f89f2
Merge "Fall back to $0 if BASH_SOURCE is unset"