Install ncclient even in check mode

When running `kayobe physical network configure --check` against Juniper
switches, Kayobe may fail with the following error:

    Failed to import the required Python library (ncclient)

This is because the ncclient installation task is skipped by check mode.

Change-Id: I643f54a67edfe22a3fd5c86f157bd1246d617399

ansible/roles/junos-switch/tasks/main.yml

@@ -11,6 +11,7 @@
     name: "ncclient{{ ncclient_version }}"
     virtualenv: "{{ lookup('env', 'VIRTUAL_ENV') | default(omit, true) }}"
   become: "{{ lookup('env', 'VIRTUAL_ENV') == None }}"
+  check_mode: false
   run_once: true
 
 - name: Ensure Juniper switches are configured

releasenotes/notes/ncclient-install-check-mode-0cfc7e846f3a978a.yaml

+---
+fixes:
+  - |
+    Installs ``ncclient`` dependency for Juniper switch configuration when
+    using Ansible check mode.