Prevent traceback when /etc/mtab is not available

* get_file_contents returns None when file is not available.
  When /etc/mtab is pointing to /proc/mounts and /proc is not
  mounted, mtab.split will traceback.
Check presence of files under /proc before opening them
Add support for running modules chrooted in a local dir

Adds 'chroot' connection for executing modules chrooted to
a local dir. Requires running ansible as root.

chroot dirs should be specified in the inventory like any
other host.

You can do things like:

$ sudo -E ansible -vvv -f 1 "./chroot1,./chroot2" -c chroot \
  all -m setup
$ sudo -E ansible-playbook -vvv -f 1 -i "./chroot1,./chroot2" \
  -c chroot some-playbook.yml

some-playbook.yml:
---
- hosts: all
  tasks:
    - name: echo something
      shell: echo "Yaaay!" >/tmp/foobar.txt
    - name: install less
      apt: pkg=less state=latest
Merge branch 'local-chroot-support-v2' of git://github.com/mmoya/ansible into testing
Fix comment in chroot connection type