==> homestead-7: Booting VM...
==> homestead-7: Waiting for machine to boot. This may take a few minutes...
homestead-7: SSH address: 127.0.0.1:2222
homestead-7: SSH username: vagrant
homestead-7: SSH auth method: private keyTimed out while waiting for the machine to boot. This means that
Vagrant was unable to communicate with the guest machine within
the configured ("config.vm.boot_timeout" value) time period.

解决方法：各种方法试遍，不行。临时方案：每次vagrant add 重新生成一次虚拟机，修改配置后使用vagrant provision或者homestead provision的方式应用配置，不重启虚拟机，一旦重启 第二次必定再也无法成功启动了！——应该是virtualbox的版本问题。