allow users to specify what username to use when ssh-ing into the vm.
If the username flag is set, the username will be the flag value. If the
flag is not set and the the vm name is specified, the default user of
the vm will be used. if the flag is not set, and the vm name is not
specified, then the username of the default connection will be used.
Signed-off-by: Ashley Cui <acui@redhat.com>
e3e9e5175f