net: coap: define default CoAP version

RFC 7252 (CoAP) specifies value of the Version (Ver) field in the
protocol header to value 1. This patch defines value of the Version
field to make packet initialization easier. All samples and tests
are updated to use the new COAP_VERSION_1 field when initializing
a CoAP packet.

Signed-off-by: Hubert Miś <hubert.mis@nordicsemi.no>