exec: always make explicit the tty value

otherwise runc will take by default the value used for creating the
container.  Setting it explicit overrides its default value and we
won't end up trying to use a terminal when not available.

Closes: https://bugzilla.redhat.com/show_bug.cgi?id=1625876

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

libpod/oci.go

@@ -741,6 +741,8 @@ func (r *OCIRuntime) execContainer(c *Container, cmd, capAdd, env []string, tty
 
 	if tty {
 		args = append(args, "--tty")
+	} else {
+		args = append(args, "--tty=false")
 	}
 
 	if user != "" {