make sure stun is pushed in a case using stun_only_option

[igorolhovskiy]

In a case if using stun_only_option, take server parameter and push it to global pjsua stun settings

[igorolhovskiy]

Just to add a bit more comment on it, on a turn section
Option stun_only is valid only in a case if enabled == false. A bit controversial, but why not. (What I can do here - is to require enabled == true at a first place in a turn settings). Let me know what you think.
This change make sure we push stun server setting globally to PJSUA per docs.
Also I removed explicit setting of PJSUA_STUN_USE_DEFAULT as it's default option and I don't see mechanism of changing STUN/ICE setting during the test.
And final remark, this option disables ICE mechanism, just making possible of getting external address via STUN mechanism, but not gathering ICE candidates.