Improve geni-revoke-member-certificate

[tcmitchell]

The certificate revocation script geni-revoke-member-certificate is hard to use. Make it easier.

The current script needs the user's certificate in a file. That requires extracting the certificate from the database. It would be easier if the user's member id (UUID) or username could be provided and the script could extract the certificate from the database. This would avoid the manual step of extracting the certificate from the database.

It would also be good if there was a man page to help with the options. Additionally if the command line help listed the default values it would make the script easier to run because most options have good defaults and do not need to be specified.