Perform a self bind mount of rootfs itself in fs::chroot::enter.

Syscall 'pivot_root' requires that the old and the new root are not in the same filesystem. Otherwise, the user will receive a "Device or resource busy" error.

Currently, we rely on the provisioner to prepare the rootfs and do proper bind mount if needed so that pivot_root can succeed. The drawback of this approach is that it potentially pollutes the host mount table which requires cleanup logics.

For instance, in the test, we create a test rootfs by copying the host files. We need to do a self bind mount so that we can pivot_root on it. That pollute the host mount table and it might leak mounts if test crashes before we do the lazy umount:
https://github.com/apache/mesos/blob/master/src/tests/containerizer/launch_tests.cpp#L96-L102

What I propose is that we always perform a recursive self bind mount of rootfs itself in fs::chroot::enter (after enter the new mount namespace). Seems that this is also done in libcontainer:
https://github.com/opencontainers/runc/blob/master/libcontainer/rootfs_linux.go#L402