Change default CA file location.

This changes the default CA filename from 'cert.pem' to 'btcd.cert' to
reflect the fact that this cert is used for securely connecting to
btcd.  With the introduction of autogenerated btcwallet certs (saved
to rpc.cert and rpc.key) this change was made to better differentiate
the two cert files.

To upgrade across this change, simply move the btcd cert.  On unix,
use the command:

$ mv ~/.btcwallet/cert.pem ~/.btcwallet/btcd.cert