Support voluntary exits via validator manager

[karalabe]

The validator_manager supports creating and importing validator keys. The creation command makes one big file containing all the keys; and the importer imports each key individually with a randomly generated password. There is no voluntary exit command via the validator_manager. This makes things a bit quirky.

• I need to point lighthouse to one of the imported keys, since I can't use the dump containing many keys.
• The only place where I have a set of keys is the validator itself, so I need to dig up the key from the live system.
• I also need to look through the validator_definitions and extract the correct random password.

It's a bit of a PITA even for a single validator. Doing this for many gets annoying very fast.

Would be nice to be able to generate voluntary exists via the validator_manager too.

[aronfoster]

Would be nice if the validator_manager also added the ability to delete keys, which you'll want to do after exiting.

[rnicoll]

Taking a look at this currently, hoping to have a first PR later this month.