Only watch salt-api package if install_packages=true

Also watch salt-api pkg instead of salt-master one.

salt/api.sls

     - name: {{ salt_settings.api_service }}
     - require:
       - service: {{ salt_settings.master_service }}
+{% if salt_settings.install_packages %}
     - watch:
-      - pkg: salt-master
+      - pkg: salt-api
+{% endif %}