providers/gce: add InstallInitScript task
Split GCE manifest into regular & backports

Some people want the security team-supported 3.2 kernel, while others
want the better performance of the backports kernel. Both flavors are
currently provided in the Google Compute Engine debian-cloud project.

Also add OpenSSH to the backports flavor, for parity with the current
GCE backports image based on build-debian-cloud. This backport is
included for performance reasons, in line with the purpose of that image
flavor, and as discussed on the debian-cloud list.
Merge pull request #92 from jkaplowitz/gce-manifest-split

Split GCE manifest into regular & backports
Merge pull request #88 from proppy/add-gce-init

add InstallInitScript task to gce provider
Fix name of chef plugin manifest schema name
Make "stable" the salt install_source default.
Fix a few indentation things
Make "assets" a required property in chef schema
plugins/docker_daemon: initial import

A plugin that install the docker daemon w/ an init script.
Add --assume-yes to apt-get autoremove
Check if qemu-img is available
Add CheckAptProxy task

Check if the specified APT proxy server can be reached, informing the
user if this can't be done. That will help them to debug the errors that
will be raised by `apt-get` because of the misleading proxy
configuration.

This closes #95.
Might as well check apt proxy early
Moved CheckAptProxy to preparation phase
Changed the logging to output a warning instead of an error
Changed the error message a little
Added pip_install plugin

configuration is like

"pip_install": {"packages": ["a_package", "another_package==1.0.1"]}

Installs build-essential and python-dev