Salt-key using external auth and token file is broken

[msciciel]

Description of Issue/Question

It's not possible to use token file with external auth:

$ salt-key -T --auth=pam
username: example
password:
Error: 'token_file'

$ ls salt_token
ls: cannot access salt_token: No such file or directory

### Setup
Enable external auth on salt-master:

external_auth:
pam:
users%:
- '.*'
- '@wheel'
- '@runner'
- '@jobs'
limited%:
- 'test.ping'
- 'state.apply'
- 'state.highstate'
- 'grains.get'
- 'grains.item'
- 'grains.items'
- 'pillar.get'
- 'pillar.item'
- 'pillar.items'
- 'pkg.list_pkgs'
- 'pkg.list_repos'
- 'pkg.version'

### Steps to Reproduce Issue
$ salt-key -T --auth=pam

### Versions Report

Salt Version:
Salt: 2017.7.6

Dependency Versions:
cffi: Not Installed
cherrypy: unknown
dateutil: Not Installed
docker-py: Not Installed
gitdb: Not Installed
gitpython: Not Installed
ioflo: Not Installed
Jinja2: 2.8.1
libgit2: Not Installed
libnacl: Not Installed
M2Crypto: Not Installed
Mako: Not Installed
msgpack-pure: Not Installed
msgpack-python: 0.4.6
mysql-python: Not Installed
pycparser: Not Installed
pycrypto: 2.6.1
pycryptodome: Not Installed
pygit2: Not Installed
Python: 2.7.14 (default, Jan 31 2018, 02:12:13)
python-gnupg: Not Installed
PyYAML: 3.11
PyZMQ: 14.5.0
RAET: Not Installed
smmap: Not Installed
timelib: Not Installed
Tornado: 4.2.1
ZMQ: 4.0.5

System Versions:
dist: centos 6.9 Final
locale: UTF-8
machine: x86_64
release: 2.6.32-696.30.1.el6.x86_64
system: Linux
version: CentOS 6.9 Final

[Ch3LL]

thanks for the PR! We really appreciate it :) since the PR resolves the issue I will close here and we can keep the discussion in the PR. Let me know if you want me to re-open here though :)