Add ability to keep local cache for debmirror's repo syncs

Add new cache_dir parameter to keep local cache for debmirror
repo and re-use it. During first run debmirror will create
cache and then using hardlinks cache will be copied to target
repo. All next runs will copy cache first and then run debmirror
with prepared cached.

Change-Id: Ib7932e64c75743b06b3b12507fe87a30d4d93311
diff --git a/tests/pillar/client.sls b/tests/pillar/client.sls
index 005dd04..6a8fd98 100644
--- a/tests/pillar/client.sls
+++ b/tests/pillar/client.sls
@@ -15,6 +15,7 @@
         dist: [ xenial ] #, xenial-security, xenial-updates ]
         section: [ main ] #, multiverse, restricted, universe ]
         exclude_deb_section: [ 'games', gnome, Xfce, sound, electronics, graphics, hamradio , doc, localization, kde, video ]
+        cache_dir: "/tmp/mirror/.cache/ubuntu"
         filter:
           00: "--exclude='/*'"  # exclude all for test..
           01: "--include='/vim-tiny'"  # and include something small.