net: openthread: Allow to disable automatic network attachment

Add OpenThread configuration option, which allows to configure and start
OpenThread stack operation manually. This mode should be used in NCP
devices, as well as is needed for certification purposes, where
OpenTread stack have to be configured by the test framework and not
initialize and join the network on its own.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>