ansible: rename "joyent" machines to "equinix_mnx"

Rename the machines to more accurately reflect where those machines
are hosted (the MNX owned Equinix Metal account).

Refs: nodejs#3108

ansible/inventory.yml

@@ -20,7 +20,7 @@ hosts:
         ubuntu1804-x64-1: {ip: 169.45.166.50, alias: ci-release}
         ubuntu2004-x64-1: {ip: 169.60.150.91, alias: ansible}
 
-    - joyent:
+    - equinix_mnx:
         debian10-x64-1: {ip: 147.28.162.110, alias: grafana}
         smartos15-x64-1: {ip: 139.178.83.227, alias: backup}
         ubuntu1604-x64-1: {ip: 147.28.162.105, alias: unencrypted}
@@ -50,7 +50,7 @@ hosts:
     - iinthecloud:
         ibmi73-ppc64_be-1: {ip: 65.183.160.62, user: nodejs}
 
-    - joyent:
+    - equinix_mnx:
         smartos18-x64-2: {ip: 147.28.162.101}
         smartos20-x64-2: {ip: 147.28.162.108}
         ubuntu1804_docker-x64-1: {ip: 147.28.162.104, user: ubuntu}
@@ -167,7 +167,7 @@ hosts:
         ubuntu1804-x64-1: {ip: 52.117.26.14, alias: jenkins-workspace-6}
         ubuntu1804-x64-2: {ip: 50.97.245.9}
 
-    - joyent:
+    - equinix_mnx:
         smartos18-x64-3: {ip: 147.28.162.102}
         smartos18-x64-4: {ip: 147.28.162.103}
         smartos20-x64-3: {ip: 147.28.162.107}