builder/virtualbox-ovf,vmware-vmx: add `boot_command` support

Fixes #1082
added docs for boot_command and boot_wait
Merge branch '1082-add-boot-commands-to-vmx-and-ovf-builders' of github.com:rasa/packer into rasa-1082-add-boot-commands-to-vmx-and-ovf-builders

Conflicts:
	builder/virtualbox/ovf/config.go
	builder/vmware/vmx/config.go
Merge branch 'rasa-1082-add-boot-commands-to-vmx-and-ovf-builders'
builder/virtualbox: support http server for ovf [GH-1169]