Allow setting CEPH_KEYRING from an option rather than an envvar

[olorin]

It's unintuitive to require runtime configuration with both CLI options and envvars (particularly as the latter aren't mentioned in the output of vault --help). Work out how to specify the Ceph keyring based on an option to the vault binary (may have to do this for inspect as well?). If there's no better way, we can just call setEnv with the provided value before we pass control to the command daemon.

[istathar]

Brilliant @fractalcat I'm glad you took this on.

(may have to do this for inspect as well?)

I daresay, yes.

AfC