Standarise tracing for scripts

There is a wide variety of tracing options through the various shell
scripts.  Some use "set -eux", others explicity set xtrace and others
do nothing.  There is a "-x" option to bin/disk-image-create but it
doesn't flow down to the many scripts it calls.

This adds a global integer variable set by disk-image-create
DIB_DEBUG_TRACE.  All scripts have a stanza added to detect this and
turn on tracing.  Any other tracing methods are rolled into this.  So
the standard header is

---
if [ "${DIB_DEBUG_TRACE:-0}" -gt 0 ]; then
    set -x
fi
set -eu
set -o pipefail
---

Multiple -x options can be specified to dib-create-image, which
increases the value of DIB_DEBUG_TRACE.  If script authors feel their
script should only trace at higher levels, they should modify the
"-gt" value.  If they feel it should trace by default, they can modify
the default value also.

Changes to pachset 16 : scripts which currently trace themselves by
default have retained this behaviour with DIB_DEBUG_TRACE defaulting
to "1".  This was done by running [1] on patch set 15.  See the thread
beginning at [2]

dib-lint is also updated to look for the variable being matched.

[1] https://gist.github.com/ianw/71bbda9e6acc74ccd0fd
[2] http://lists.openstack.org/pipermail/openstack-dev/2014-November/051575.html

Change-Id: I6c5a962260741dcf6f89da9a33b96372a719b7b0

elements/select-boot-kernel-initrd/bin/select-boot-kernel-initrd

@@ -15,6 +15,9 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
+    set -x
+fi
 set -eu
 set -o pipefail
 

elements/select-boot-kernel-initrd/cleanup.d/99-remove-dib-img-functions

@@ -1,5 +1,8 @@
 #!/bin/bash
 
+if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
+    set -x
+fi
 set -eu
 set -o pipefail
 

elements/select-boot-kernel-initrd/extra-data.d/99-copy-dib-img-functions

@@ -1,6 +1,9 @@
 #!/bin/bash
 # Copy elements-functions into chroot so we can access the get_kernel_image function
 
+if [ ${DIB_DEBUG_TRACE:-0} -gt 0 ]; then
+    set -x
+fi
 set -eu
 set -o pipefail