packer: use pip instead of easy_install

packer/build-vm-bootstrap.yml

@@ -10,8 +10,10 @@
 ### Build instances 
 
     - shell: mkdir -p log/
-    - easy_install:
+    - name: Install latest version of slapos.libnetworkcache
+      pip:
         name: slapos.libnetworkcache
+        state: latest
 
     - shell: PATH=$PATH:/opt/packer/ packer build -var 'disk_size={{ item.size }}' -var 'name={{ item.name }}' -var 'custom_script={{ item.script }}' debian9.json >> log/debian9-{{ item.name }}.log
       args: