Support user variables for missing OpenStack string config options

Fixes #1506 

Boot command section in vmware and virtualbox ISO wasn't working
[skip ci]

builders/googlecompute: made formatting consistent

Also updated Debian 7 VM image to `debian-7-wheezy-v20150127`.

builders/googlecompute: fixed link in docs.

Clarify error message in case of an atlas client connection failure

When a packer json contains an atlas postprocessor, and `packer validate` runned locally
in case of the missing ATLAS_TOKEN env var, the following error message is displayed:

* Error initializing client: authentication failed

It wasn't clear which plugin produced this message, so the amazon-ebs builder was the suspect

Add Atlas post-processor documentation

Update Push APIs

This allows children to get the raw templates without re-reading the
file.

Fixes #1881 Don't include Windows Disks when packaging boxes for prl

Boot command section was NOT working

Boot command section was NOT working

Updating from official repo

If copy and pasted, the boot command section was NOT working due to < and > being translated to HTML code equivalents. Removed javascript tagging on the code block to see if this fixes the resulting display in a browser.

If copy and pasted, the boot command section was NOT working due to < and > being translated to HTML code equivalents. Removed javascript tagging on the code block to see if this fixes the resulting display in a browser.

This excludes the directory "Windows Disks" present in the VM data
directory if you hava a Windows VM with Parallels tools installed.

Add Travis CI badge to README
[skip ci]

Fix page title
[skip ci]

Use the new Travis Docker infrastructure and stuff