Improve ability to manage keys through CLI

[effenkeith]

It is currently possible to remove the public key from the local Singularity keyring. Can this feature be added to remove the private keyring as well?

Deleting the files associated with the keyring seems like a brute-force method and does not take into consideration management of multiple keys.

~/.singularity/sypgp/pgp-secret ~/.singularity/sypgp/pgp-public

There is a placeholder for this type of effort, to make the design and implementation more user friendly and expressive. https://github.com/sylabs/singularity/issues/79