Merge lp:~james-page/charm-helpers/fix-apt-race into lp:charm-helpers

[James Page]

Fixup apt race

=== modified file 'charmhelpers/core/host.py'
--- charmhelpers/core/host.py	2014-06-26 13:42:36 +0000
+++ charmhelpers/core/host.py	2014-07-23 11:19:58 +0000
@@ -322,6 +322,10 @@
     import apt_pkg
     if not pkgcache:
         apt_pkg.init()
+        # Force Apt to build its cache in memory. That way we avoid race
+        # conditions with other applications building the cache in the same
+        # place.
+        apt_pkg.config.set("Dir::Cache::pkgcache", "")
         pkgcache = apt_pkg.Cache()
     pkg = pkgcache[package]
     return apt_pkg.version_compare(pkg.current_ver.ver_str, revno)