added default value of 4096 bits to config for master and minion RSA-Key

Hey,

While testing the salt-mine i discovered, that the authentication phase for minions in larger setups (7000 minions, single master) can become quite expensive cpu-wise for the salt-master. About a 1000 minions reporting values via the salt-mine can raise the load of a DELL R610, 16 Cores, 16GB Ram, etc. to about 6-10. Having more minions report, would certainly kill the master sooner or later.

Salt uses 4096 bit keys per default (hardcoded in crypt.py). After researching a bit i think, that 2048 bit are still fine for most use-cases. The NIST and other sources claim, that 2048 bit keys  will be fine for the next few years.
Im not a crypto-guy, im relying on a few articles here. 

After some performance testing with different key-sizes (mainly 2048 and 4096) i discovered, that using 2048 bit keys is about 6 times as fast compared to 4096 bit keys. The skript used is this one:

https://github.com/felskrone/utils/blob/master/key-perf.py

$ key-perf.py <keysize> <decryptions>

msg: "test fusel"
keysize: 2048 bit
decryptions: 5000
duration: 1:28 min

msg: "test fusel"
keysize: 4096 bit
decryptions: 5000
duration: 7:43 min

I have not done any testing with 2048 bit keys within salt besides checking that the master also can handle 2048 bit keys. If the authentication-performance will increase by as much as my test-skript, for some it might be a real option to use 2048 bit keys.

Even though im not sure i'll be using 2048 bit keys in the future, i think being able to configure it would be nice.