otto fails to detect correct file format after 'file' updated on trusty

[Francis Ginther]

The 'file' package was updated on trusty on 20131206 (https://launchpad.net/ubuntu/+source/file).

It now reports a different result then expected when examining ISO files:
ubuntu@dx-autopilot-nvidia:/tmp/fginther$ cksum trusty-desktop-i386.iso
1946923001 945815552 trusty-desktop-i386.iso
ubuntu@dx-autopilot-nvidia:/tmp/fginther$ file -L -b trusty-desktop-i386.iso
x86 boot sector

The same file on q-jenkins:
fginther@jatayu:/Data/ISOs/trusty$ cksum trusty-desktop-i386.iso
1946923001 945815552 trusty-desktop-i386.iso
fginther@jatayu:/Data/ISOs/trusty$ file -L -b trusty-desktop-i386.iso
# ISO 9660 CD-ROM filesystem data 'Ubuntu 14.04 LTS i386 ' (bootable)

update_host is now failing with:
2013-12-08 00:08:11,511 INFO Creating container 'trusty-i386-20131208-0008'
2013-12-08 00:08:11,513 ERROR image '/var/lib/lxc/trusty-i386-20131208-0008/trusty-desktop-i386.iso' is not an iso9660
Traceback (most recent call last):
  File "/var/lib/jenkins/bin/otto", line 50, in <module>
    sys.exit(cmd.run())
  File "/var/lib/jenkins/otto/ottolib/commands.py", line 168, in cmd_create
    local_config=self.args.local_config)
  File "/var/lib/jenkins/otto/ottolib/container.py", line 104, in create
    self._mountiso(container_imagepath)
  File "/var/lib/jenkins/otto/ottolib/container.py", line 372, in _mountiso
    (isomount, squashfs) = utils.get_iso_and_squashfs(container_imagepath)
TypeError: 'NoneType' object is not iterable

Example:
http://q-jenkins.ubuntu-ci:8080/job/autopilot-trusty-setup_otto/45/label=qa-intel-4000/console