feat #0008967: Allow to set a leader on a project using XML-RPC API

[atisne]

Add new services to be able to affect a user as test leader on a project.

[squash-labs]

Manage this branch in Squash

Test this branch here: https://atisnefeature-set-leader-at-pr-3qb1g.squash.io

[fmancardi]

Please use indent with 2 spaces, this is the TestLink standard, no matter what PHP standards sid

[fmancardi]

Please use always {}

NO GOOD
if ($password)
$res = $this->userMgr->setPassword($password);

            if ($password) {
                $res = $this->userMgr->setPassword($password);
           }