Allow multiple nics to be connected to VM

[remi-espie]

This allows Packer to create and allocate multiple nics to a VM through switches_names array. switch_name is still available to ensure backward compatibility. It also adds mac_addresses array, which will give the first element as a mac address to the first switch of switches_names, the 2nd element to the 2nd switch, etc.

This is quite bare bone, only creating, adding and deleting (if needed) multiple nics the the VM. This answer some of #86 needs but not all of them so I'm not marking it as closed.

Personally tested and working in the following cases:

• [x] only switch_name is set (no switches_names are set)
• [x] only switches_names are set (no switch_name is set)
• [x] one or multiple switches_names are set
• [x] already existing switches are not deleted
• [x] no mac addresses are set
• [x] less mac addresses are set than switches
• [x] more mac addresses are set than switches

[hashicorp-cla]

All committers have signed the CLA.

[remi-espie]

sorry about both last commits, some git mistakes by my side. They were removed by a push force, don't take them into account.

[galthaus]

Hi All, I need this too. I'm working on an update that implements what the commenters suggested as the better way.

It is much larger change. I'll submit it as a separate PR against main.

Thanks, Greg