runc exec: don't enable terminal unless -t is set

If container's config.json have `"terminal": true` setting in its
"process" section, runc exec assumes that stdin (fd 0) is a terminal
and tries to use it.

This leads to the following error in case stdin is not a terminal:

> ERRO[0000] exec failed: provided file is not a console

So, even if -t/--tty is not set, exec uses stdin as a terminal.
It does not help that urfave/cli v1 parser we use does not allow
to use `-t no` or `-t false`.

Since the settings in config.json is probably for the container run/start,
not for the auxiliary process started inside a container with exec, do
not use a setting from there, only treating stdin as a terminal in case
`-t` is explicitly given.

Tests that use runc exec with a terminal are amended with -t.

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

exec.go

@@ -203,6 +203,7 @@ func getProcess(context *cli.Context, bundle string) (*specs.Process, error) {
 	p.Env = append(p.Env, context.StringSlice("env")...)
 
 	// set the tty
+	p.Terminal = false
 	if context.IsSet("tty") {
 		p.Terminal = context.Bool("tty")
 	}