net: openthread: configure required masterkey

After the latest upmerge, OpenThread requires explicit configuration
of the Master Key. This commit adds a Kconfig symbol that can be
used to setup its value. By default no Master Key is configured and
OpenThread generates a random one.

The Sockets Echo samples are configured with a fixed key with this
commit.

Signed-off-by: Eduardo Montoya <eduardo.montoya@nordicsemi.no>