[CLI] shared-recovery create: make threshold optional if recipients is not specified

[AureliaDolo]

Threshold arg should not be required when recipients are not specified (it should have the same requires = "recipients", as weights)

Default recipients are all admins expect author.

Default for threshold could be

• max(1, ⌊admin_count * k⌋) with k between 0 and 1.

What should be k default value, should it be configurable ? (no)

I'd rather go for 2/3

[mmmarcos]

Discussed with @vxgmichel and @touilleMan: instead of having a default threshold, the CLI could prompt the user for the threshold value to be used.