builder/parallels: Ignore 'The fdd0 device does not exist'

A current floppy disk probably doesn't exist, so the build shouldn't
fail when we try to delete it.
Fixed dead links in doc section environment-variables.

Due to a missing `/` the links pointed to i.e. `docs/other/docs/other/core-configuration.html` producing a 404 / Page not found.
builder/parallels: Init VM without hdd and attach it later

Avoid the disk resize because it is unsupported for some guests (OS X)
http://kb.parallels.com/en/113653
Remove unnecessary counter file (as per @pearkes)
Update website to use latest middleman
Merge pull request #1501 from JessThrysoee/current_fdd0_does_not_exist

builder/parallels: Ignore 'The fdd0 device does not exist'
Merge pull request #1554 from mitchellh/sethvargo/update_middleman

Update website to use latest middleman
builder/parallels: Rely on Cleanup functions to detach devices

Remove the redundant StepRemoveDevices and rely on the cleanup to be handled by:
* StepAttachParallelsTools.Cleanup
* StepAttachFloppy.Cleanup
* stepAttachISO.Cleanup
builder/parallels: Added driver method 'DeviceAddCdRom'
builder/parallels: Attach ISO images to the separate cdrom device

For each ISO image the individual cdrom device will be added to the VM. During the cleanup these devices will be deleted.
It makes attach steps more clear - there is no doubt what is the name of the device.

Related to: [mitchellh/packer#1502]
builder/parallels: Added setting the boot order

CD/DVD drive with installation ISO should be exactly the 2nd device in the VM boot order.
This ensures that VM will boot from this ISO at the first boot time only.
builder/parallels: Disable default cdrom0 device and then enable it on cleanup

This is needed to make 'cdrom0' device unavailable during the ISO installation process.
It is required for some guest OS types, such as FreeBSD and OmniOS.
Merge pull request #1502 from JessThrysoee/remove_step_remove_devices

builder/parallels: Rely on Cleanup functions to detach devices
Merge pull request #1548 from legal90/create_disk

builder/parallels: Create VM without hdd and then add it later
Updated CHANGELOG
fix: Respect metadata supplied for GCE builders.

Previously, any per instance metadata supplied via the GCE builder
was ignored.

Test plan:

- make test
- Manual testing via:
-- Created a packer config that contained a GCE builder with custom
   metadata set.
-- Ran `packer build`.
-- Verified the instance had the correct metadata in the GCE console.
Move assets to assets/
Upgrade to middleman-hashicorp
Tell Rack to cache /assets
Merge pull request #1564 from mitchellh/sethvargo/middleman-hashicorp

Use middleman-hashicorp