Armory needs bitcoind to auto manage. Ima go ahead and assume what you have in /usr/bin is BitcoinQt. Do a whereis bitcoind, and point Armory to that.
Or do not point it to anything and let it look at default locations, it is usually quite efficient at that on Ubuntu.
Armory couldn't find the Bitcoin installation, so I installed Bitcoin through Armory. Then it found the installation. But every time I open Armory, it rebuilds and rescans the database. Even if it completely rebuild&rescanned, if it close Armory, and open it again, it will R&R again.