Before this PR, people can set listen-client-urls without setting
advertise-client-urls, and leaves advertise-client-urls as default
localhost value. The client libraries which sync the cluster info
fetch wrong advertise-client-urls and cannot connect to the cluster.
This PR avoids this case and provides better UX.
On the other hand, this change is safe because people always want to set
advertise-client-urls if listen-client-urls is set. The default localhost
advertise url cannot be accessed from the outside, and should always be
set except that etcd is bootstrapped with no flag.
Support IPv6 address for ETCD_ADDR and ETCD_PEER_ADDR
pkg/flags: Support IPv6 address for ETCD_ADDR and ETCD_PEER_ADDR
pkg/flags: tests for IPv6 addr and bind-addr flags
pkg/flags: IPAddressPort.Host: do not enclose IPv6 address in square brackets
pkg/flags: set default bind address to [::] instead of 0.0.0.0
pkg/flags: we don't need fmt any more
also, one minor fix: net.JoinHostPort takes string as a port value
pkg/flags: fix ipv6 tests
pkg/flags: test both IPv4 and IPv6 addresses in TestIPAddressPortString
etcdmain: test: use [::] instead of 0.0.0.0
Yicheng Qin