Add a command to start a rekey

[jaxxstorm]

This should be a subcommand of the overall rekey command:

locksmith rekey start.

It should take the following parameters:

• shares
• threshold
• pgp_keys
• whether to backup the keys or not

It will return a nonce, which should be output to be shared. In future, we can store this somewhere like consul

[jaxxstorm]

So, couple things.

1) the rekey endpoint seems to be unauthed. Anyone can start a rekey, which is nice. 2) we only need to submit to 1 vault in a datacenter. We should either use a goroutine to do all if we need to, or close it down with the datacenter flag. We need to pick a random vault server from a DC we choose.

A future enhancement would be store the rekey nonce in consul, but we can get there later.