Do not print warn message when keep alive time is unset

[nak3]

The warning message is always printed when keep alive time is unset. The current example config also causes validation error as:

$ ./bin/xds-relay  -m validate --aggregation-rules example/config-files/aggregation-rules.yaml -c example/config-files/xds-relay-bootstrap.yaml
{"level":"info","ts":1609554826.4375143,"logger":"upstream_client","msg":"Initiating upstream connection","json":{"address":"0.0.0.0:18000"}}
{"level":"warn","ts":1609554826.4375317,"msg":"Keepalive time parsing failed%!(EXTRA string=time: invalid duration \"\")","json":{},"stacktrace":"github.com/envoyproxy/xds-relay/internal/pkg/log.(*logger).Warn\n\t/home/knakayam/dev/xds-relay/internal/pkg/log/zap.go:95\ngithub.com/envoyproxy/xds-relay/internal/app/upstream.getKeepaliveParams\n\t/home/knakayam/dev/xds-relay/internal/app/upstream/client.go:379\ngithub.com/envoyproxy/xds-relay/internal/app/upstream.New\n\t/home/knakayam/dev/xds-relay/internal/app/upstream/client.go:107\ngithub.com/envoyproxy/xds-relay/internal/app/server.RunWithContext\n\t/home/knakayam/dev/xds-relay/internal/app/server/server.go:95\ngithub.com/envoyproxy/xds-relay/internal/app/server.Run\n\t/home/knakayam/dev/xds-relay/internal/app/server/server.go:46\nmain.glob..func1\n\t/home/knakayam/dev/xds-relay/main.go:45\ngithub.com/spf13/cobra.(*Command).execute\n\t/home/knakayam/.go/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:846\ngithub.com/spf13/cobra.(*Command).ExecuteC\n\t/home/knakayam/.go/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:950\ngithub.com/spf13/cobra.(*Command).Execute\n\t/home/knakayam/.go/pkg/mod/github.com/spf13/cobra@v1.0.0/command.go:887\nmain.main\n\t/home/knakayam/dev/xds-relay/main.go:67\nruntime.main\n\t/usr/local/go/src/runtime/proc.go:204"}

Hence this patch stops warning message when keep alive is unset and the default value (5m) is used.

Signed-off-by: Kenjiro Nakayama nakayamakenjiro@gmail.com