Add support for user passwords that cannot be used to login

[floatingstatic]

This is a similar patch to what was submitted to pyeapi: arista-eosplus/pyeapi#227

Modern versions of EOS allow one to specify a password that cannot be used to login which is useful for exclusive ssh-key auth:

vEOS(config)#username testuser secret ?
  *       Specify a password that cannot be used to login
  0       Specify an UNENCRYPTED password will follow
  5       Specify an ENCRYPTED MD5 password will follow
  LINE    The UNENCRYPTED (cleartext) password
  sha512  Specifies an ENCRYPTED SHA512 password will follow

Currently goeapi has no support for this. This change implements support for a nologin encryption type to support setting this feature via the api and to keep parity with changes accepted in pyeapi.