net: tls: Add socket option to set DTLS role

Add write-only socket option to set role for DTLS connection. This
option is irrelevant for TLS connections.

This options accepts and integer with a TLS role, compatible with
mbedTLS values:
0 - client,
1 - server.

By default, DTLS will assume client role.

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