manager: try to find a valid IP2IP account when IP is called

This patch fixes broken Manager::newOutgoingCall() when an IP
url is given.
Now, if the given prefered account is not a valid IP2IP like account,
ie doesn't have an empty hostname, we try to find one in all SIPAccount.
If nothing is found the method returns nullptr.

Change-Id: I60eaf99a13c8f25df8decd87286d857f75391231
Tuleap: #448