Enable PSK security mode by default

[MarceloSalazar]

During 1.2.2EA OOB, we've found out that the Cloud Client end-node doesn't connect to the BR, unless PSK mode is enabled.
This has to be enabled by default.

@SeppoTakalo @yogpan01 @JanneKiiskila @ashok-rao

[SeppoTakalo]

Uhm.. I feel like this PR is then in the wrong place.
We either modify cloud client to support non-secure mode, or if we modify this, then we need to modify all others to match.

[MarceloSalazar]

I understand that security is a must have in our offering.
What 'others' (apps?) are you referring to?
The spirit config already supports PSK by default.

[JanneKiiskila]

Example apps configs must have the same security turned on & the key must match.

[juhhei01]

Have you fix app that border router support also 64-bit address mode? Using short address is very dangerous if you set multiple router's up. Also PAN-id should be random.

[MarceloSalazar]

Discussion continues in ONME-3160