Allow ELEMENTS_DIR to be configurable

Prior to version 2.0.0 of diskimage-builder, the elements dir was hard
coded as 'elements' in dib-lint which allowed dib-lint to be consumed by
other projects (instack-undercloud). The 2.0.0 release has changed this
path and it is not overridable so that the other projects can still use
dib-lint. This change makes ELEMENTS_DIR and LIB_DIR check for an
existing environment variable so that dib-lint can be consumed outside
of the diskimage-builder project.

Change-Id: Iae1dade1e20a950e63ffe8dacfea0f23cf9923a0
Related-Bug: #1672730

bin/dib-lint

@@ -21,8 +21,8 @@
 set -eu
 set -o pipefail
 
-ELEMENTS_DIR=diskimage_builder/elements
-LIB_DIR=diskimage_builder/lib
+ELEMENTS_DIR=${ELEMENTS_DIR:-diskimage_builder/elements}
+LIB_DIR=${LIB_DIR:-diskimage_builder/lib}
 
 parse_exclusions() {
     # Per-file exclusions