Allow configuration of client SCEP algorithms
* Allow users to set `scep_cipher` and `scep_digest` in their CA
configuration. These settings are authoritative and will override
anything from the server. This was added to support connections to
systems, such as Dogtag, that do not provide a CA capabilities string
and, therefore, are prone to causing incorrect ciphers to be used on the
client side.
* In accordance with the latest SCEP Draft RFC, the default cipher has
been changed to AES-256 and the default digest has been changed to
SHA-256. These were chosen as reasonable defaults for most users and
systems.
* To ease the determination of which configuration file controls what
CA, the output of `getcert list-cas -v` was updated to print a
`config-path` entry which will list the specific configuration
associated with a given CA.
Closes #89