net: openthread: Do not overwrite stored dataset with defaults

This commit prevents a situation when stored and possibly modified
commissioner dataset is overwritten with default configuration during
OpenThread initialization.

It introduces a new function, openthread_start, which verifies if the
dataset is already stored, and if not, depending on configuration,
preloads the default configuration or initiates the join procedure.

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