Install collectd packages only when needed

This change ensures that the python-pymemcache package which is required
by collectd isn't installed if collectd itself isn't defined.
diff --git a/memcached/map.jinja b/memcached/map.jinja
index 7d8a2ad..984011f 100644
--- a/memcached/map.jinja
+++ b/memcached/map.jinja
@@ -1,6 +1,7 @@
 {% set server = salt['grains.filter_by']({
     'Debian': {
-        'pkgs': ['memcached', 'python-memcache', 'python-pymemcache'],
+        'pkgs': ['memcached', 'python-memcache'],
+        'collectd_pkgs': ['python-pymemcache'],
         'service': 'memcached',
         'config': '/etc/memcached.conf',
         'config_template': 'salt://memcached/files/memcached.conf',
diff --git a/memcached/server.sls b/memcached/server.sls
index 8ff7409..cac6923 100644
--- a/memcached/server.sls
+++ b/memcached/server.sls
@@ -6,6 +6,12 @@
   pkg.installed:
   - names: {{ server.pkgs }}
 
+{%- if pillar.collectd is defined %}
+collectd_packages_for_memcached:
+  pkg.installed:
+  - names: {{ server.collectd_pkgs }}
+{%- endif %}
+
 memcached_config:
   file.managed:
   - name: {{ server.config }}