fix(NODE-1465): Update fetch-mgmt-mac (#1422)

This commit broke nested tests/hourly pipeline:
cb9249d

This PR fixes that. Tested on nested registration test

[NODE-1465](https://dfinity.atlassian.net/browse/NODE-1465)

[NODE-1465]:
https://dfinity.atlassian.net/browse/NODE-1465?atlOrigin=eyJpIjoiNWRkNTljNzYxNjVmNDY3MDlhMDU5Y2ZhYzA5YTRkZjUiLCJwIjoiZ2l0aHViLWNvbS1KU1cifQ

ic-os/components/hostos-scripts/misc/fetch-mgmt-mac.sh

@@ -9,7 +9,7 @@ source /opt/ic/bin/logging.sh
 source /opt/ic/bin/metrics.sh
 
 SCRIPT="$(basename $0)[$$]"
-CONFIG="/boot/config/config.ini"
+DEPLOYMENT="${DEPLOYMENT:=/boot/config/deployment.json}"
 
 # Get keyword arguments
 for argument in "${@}"; do
@@ -30,16 +30,6 @@ Arguments:
     esac
 done
 
-function read_variables() {
-    # Read limited set of keys. Be extra-careful quoting values as it could
-    # otherwise lead to executing arbitrary shell code!
-    while IFS="=" read -r key value; do
-        case "$key" in
-            "mgmt_mac") mgmt_mac="${value}" ;;
-        esac
-    done <"${CONFIG}"
-}
-
 # Fetch the management MAC address of the physical machine.
 # The management MAC address will be used as unique key for:
 #  - Hostnames
@@ -67,12 +57,15 @@ function fetch_mgmt_mac() {
 
 function main() {
     # Establish run order
-    mgmt_mac=""
-    read_variables
-    if [ "${mgmt_mac}" == "" ]; then
+
+    # NOTE: `fetch-property` will error if the target is not found. Here we
+    # only want to act when the field is set.
+    MGMT_MAC=$(jq -r ".deployment.mgmt_mac" ${DEPLOYMENT})
+
+    if [ -z "${MGMT_MAC}" ] || [ "${MGMT_MAC}" = "null" ]; then
         fetch_mgmt_mac
     else
-        echo "${mgmt_mac}"
+        echo "${MGMT_MAC}"
     fi
 }