Use random UUID per default

[plajjan]

We are doing a sort of zero touch provisioning on juniper devices and noticed the serial number is the same on all instances. According to https://forums.juniper.net/t5/vSRX/changing-regenerate-vSRX-chassis-serial/td-p/316519 it appears the serial number is based on the UUID of the VM, so perhaps by changing that we get unique serial numbers.

On SROS we set the UUID of the VM based on the license file. I think we can have similar functionality for other platforms, or like a --uuid argument. In absence of this argument, I think it makes sense to use a random UUID.

I don't think we need to limit this to Juniper devices, we can have random UUID as the default for all types.

[emjemj]

Sounds reasonable.

[monrad]

I had made a draft pull request in #258 that I can expand on if that way makes sense.