providers/gce: add InstallInitScript task
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
Merge pull request #106 from Jimdo/pip_install_squashed

pip_install plugin
Update volume image path after move it

The `loopback.MoveImage` task wasn't updating `info.volume.image_path`
after moving the volume image file. That made `volume.Delete` to fail
when called, because it would try to remove a file that didn't exist
anymore.
Add `volume.Delete` to GCE taskset

This closes #97.
Save downloaded `gsutil` tarball to workspace

`gsutil` tarball was being downloaded to the current working directory
and wasn't removed after its extraction.

This will be useful until #87 is merged.
Improve ami listing performance

 * no need to list all available ami's, just self owned should be enough
Merge pull request #107 from zeridon/ami-list-performance

Improve ami listing performance
Allow stable/oldstable on manifest

This closes #94.
Support executing commands in a specific cwd

Simply plumbed through to Popen(), which already supports this.

Change-Id: If1fdf0a33c96f3aad42407fdc7c9c9f7d4b95c00