Add fallback mechanism for platforms without attach support

Some platforms (most notably CentOS and RHEL) use a kernel without
'attach' support. This patch detects this absence and falls back
to the alternative ways of doing things like detection of IP address
and halting the container.
It does so by running the command `true` through lxc-attach and testing
if that works. The `true` command can probably be safely assumed to
be available on all GNU/Linux installations. 

This patch essentially enables support for the CentOS 6 and RHEL 6
platform, at least for the basic vagrant commands `up`, `halt` and `destroy`.