Allow setting ROOT_LABEL from environment

--root-label was added with I596104d1a63b5dc6549e8460a1ae3da00165ef04

This sets the ROOT_LABEL environment variable.

Over the years how this deploys has become more complex; now this
value gets written into DIB_BLOCK_DEVICE_PARAMS_YAML default values,
which is then loaded into DIB_ROOT_LABEL.

To override this from the environment you need to specify a full
DIB_BLOCK_DEVICE_CONFIG -- we don't have a way to just merge in the
root label setting.

Using the command-line argument is difficult with tools like nodepool
where the command-line is baked into something else.  However we
already have methods for overriding environment variables on dib
calls.

Several of the other variables here accept default values from the
environment, so this is not an outlier.  Making ROOT_LABEL also do
this allows us to test with non-default root devices in the gate (see
the linked change).

Change-Id: Ia1ef48c24841a86f387ff9603c64fd23d8670193
Needed-By: https://review.opendev.org/c/zuul/nodepool/+/853574

diskimage_builder/lib/disk-image-create

@@ -126,7 +126,7 @@ INSTALL_PACKAGES=""
 IMAGE_TYPES=("qcow2")
 COMPRESS_IMAGE="true"
 DIB_GZIP_BIN=${DIB_GZIP_BIN:-"gzip"}
-ROOT_LABEL=""
+ROOT_LABEL="${ROOT_LABEL:-}"
 DIB_DEFAULT_INSTALLTYPE=${DIB_DEFAULT_INSTALLTYPE:-"source"}
 MKFS_OPTS=""
 ACI_MANIFEST=${ACI_MANIFEST:-}