vSphere upload hangs due to DOMAIN\account format of username

VMware vCenter can be integrated with Microsoft Active Directory, so usernames have format `DOMAIN\account`.
At the moment Packer hangs on image uploading, because OVFtool cannot correctly process a back-slash in the username.

This pull request makes URL-encoding for the username value, but leaves all other parameters as they are.
Also it adds a new log line to make OVFtool debugging easier.