Reimplement lxc-ps

Instead of calling lxc-ps in top endpoint, we reimplement it by
calling ps and filter for pids running in a given container.
Fix and re-enable TestGetContainersTop
Purge more hack references to Go 1.1.2 (since it requires backported archive/tar patches now, and Go 1.2 is _widely_ packaged successfully)
Merge pull request #3168 from discordianfish/2464-replace-lxc-ps

Reimplement lxc-ps
Merge pull request #3239 from tianon/old-go-not-supported

Purge more hack references to Go 1.1.2