playbook: Enforce soft and instance directories existence

As package provided does not ensure creation of /opt/slapgrid and
/srv/slapgrid add them if not present.

playbook/roles/slapos-proxy/tasks/main.yml

+  - name: Ensure software directory exists
+    file:
+      path: /opt/slapgrid
+      state: directory
+      mode: 0755
+
+  - name: Ensure instance directory exists
+    file:
+      path: /srv/slapgrid
+      state: directory
+      mode: 0755
 
   - name: Install packages using apt
     apt: name=slapos-node state=present update_cache=yes cache_valid_time=3600