Save passwords set via 'set' as hash, if asked to.

Trocla doesn't save the hashed password in the data file, even if the
format passed to 'set' is not 'plain':

$ echo foobar | trocla set testuser sha512crypt -p

$ grep -A1 testuser trocla_data.yaml
testuser:
  sha512crypt: foobar
Merge pull request #8 from yath/master

[BUG] Store hashed password supplied via 'set' instead of plaintext