As part of the larger plan to drop --cloud-provider and --cloud-config
from kube-apiserver, we need to stop calling Cloud Provider API to
find the external ip address when one is not specified on the command
line.

When ExternalHost is not specified, we check if AdvertiseAddress is
specified and use that, if that is missing then we use os.Hostname().

When testing this feature, found a problem that when ExternalHost
is specified, the port was not added in the generated URL. So fixed
that as well.