Arvo testnets typically use Ropsten's deploy of the Azimuth contract
instead of mainnet's. This commit adds an 'arvo-ropsten' derivation for
Arvo as specialised to Ropsten.
In addition to using a different Azimuth address in %zuse, this also
configures %ames and :acme appropriately for running an alternate
network on Ropsten. %ames has its protocol version incremented, and
:acme uses the Let's Encrypt staging API.
Included here are derivations for creating brass and ivory pills using
arvo-ropsten, to boot.
