Update linphone-sdk and submodule to force python3

Update cmake scripts to force python3. Update docker images to install python3 and required python3 modules when it was not already done.

See :

• linphone-cmake-builder!156 (merged)
• BC/public/external/decaf!15 (closed)
• liblinphone!1169 (merged)
• mediastreamer2!377 (merged)