[App Service]az appservice plan create: Add default OS as Linux

src/azure-cli/azure/cli/command_modules/appservice/_params.py

@@ -126,7 +126,8 @@ def load_arguments(self, _):
 subscription than the app service environment, please use the resource ID for --app-service-environment parameter. ",
                    local_context_attribute=LocalContextAttribute(name='ase_name', actions=[LocalContextAction.GET]))
         c.argument('sku', arg_type=sku_arg_type)
-        c.argument('is_linux', action='store_true', required=False, help='host web app on Linux worker')
+        c.argument('is_linux', arg_type=get_three_state_flag(), default=True, required=False,
+                   help='Host web app on Linux worker. Defaults to true. Use "--is-linux false" to create a Windows plan.')
         c.argument('hyper_v', action='store_true', required=False, help='Host Windows Container Web App on Hyper-V worker.')
         c.argument('per_site_scaling', action='store_true', required=False, help='Enable per-app scaling at the '
                                                                                  'App Service plan level to allow for '

src/azure-cli/azure/cli/command_modules/appservice/_validators.py

@@ -109,6 +109,9 @@ def _validate_asp_sku(sku, app_service_environment, zone_redundant):
 
 def validate_asp_create(namespace):
     validate_tags(namespace)
+    # When --hyper-v is set, override is_linux to False (Windows container plan)
+    if namespace.hyper_v:
+        namespace.is_linux = False
     if namespace.sku is None:
         if namespace.is_linux:
             namespace.sku = 'P0V3'