Adding docs to #645 on the website

Setting `"ami_groups": ["all"]` will make the AMI publicly accessible.
Correcting website docs for this.
Merge pull request #894 from vogxn/docfix-ami-groups

Adding docs to #645 on the website
Add import_opts to the virtualbox-ovf builder. My use case is that I
have existing virtualbox virtual machines where I need to keep the mac
addresses so that the network interfaces come up when packer boots them.
Otherwise I just get SSH timeout and I'm unable to configure the machines.
Merge branch 'bjj-add-importopts' of github.com:b2jrock/packer into bjj-add-importopts