030-armada-boostrap.sh: make --target-manifest env var driven

Currently the manifest targeted by armada in the
030-armada-bootstrap.sh script is hard-coded to "ucp-bootstrap". This
patch set updates the 030 script to make drive the target manifest
off of a new environment variable "TARGET_MANIFEST". By default
"ucp-bootstrap" is still used, and existing functionality should not
be altered.
The motivation behind this, is it allows us to use the armada
bootstrap script to do more than just bootstrap the ucp components,
which may prove useful in zuul gate jobs that use treasuremap.

Change-Id: If3cf11f0d5701e273f5e626bff673a2b164284e3

tools/deployment/airskiff/developer/030-armada-bootstrap.sh

@@ -20,6 +20,7 @@ set -xe
 : "${INSTALL_PATH:="$(pwd)/../"}"
 : "${PEGLEG:="./tools/airship pegleg"}"
 : "${PL_SITE:="airskiff"}"
+: "${TARGET_MANIFEST:="ucp-bootstrap"}"
 
 # Render documents
 ${PEGLEG} site -r . render "${PL_SITE}" -o airskiff.yaml
@@ -30,4 +31,4 @@ docker run --rm --net host -p 8000:8000 --name armada \
     -v "$(pwd)"/airskiff.yaml:/airskiff.yaml \
     -v "${INSTALL_PATH}":/airship-components \
     quay.io/airshipit/armada:latest \
-    apply /airskiff.yaml --target-manifest ucp-bootstrap
+    apply /airskiff.yaml --target-manifest $TARGET_MANIFEST