Fix bitcoin.conf path when using with tesnet

Bitcoind and Armory configurations for rpc calls were different when
running in testnet mode:
Since we don't explicitly tell bitcoind what configuration file we want
to use, he takes the default one under ~/.bitcoin/bitcoin.conf.
Armory, however was using ~/.bitcoin/testnet3/bitcoin.conf.
This issue prevented any communication between Armory and bitcoind.
Solved by using default bitcoind config path for testnet as well.