nmcli: fixed inability to change mtu on vlan connection (#6104)

* tests updated

Co-authored-by: Sam Potekhin <heaveaxy@gmail.com>

changelogs/fragments/4387-nmcli-mtu-for-vlan-connection-fix.yml

@@ -0,0 +1,2 @@
+bugfixes:
+  - nmcli - implemented changing mtu value on vlan interfaces (https://github.com/ansible-collections/community.general/issues/4387).

plugins/modules/nmcli.py

@@ -1772,6 +1772,7 @@ class Nmcli(object):
             'dummy',
             'ethernet',
             'team-slave',
+            'vlan',
         )
 
     @property

tests/unit/plugins/modules/test_nmcli.py

@@ -663,6 +663,7 @@ ipv6.method:                            auto
 ipv6.ignore-auto-dns:                   no
 ipv6.ignore-auto-routes:                no
 vlan.id:                                10
+802-3-ethernet.mtu:                     auto
 """
 
 TESTCASE_VXLAN = [