[hostcfgd] -- Fix the default for failthrough as false.

This implies that by default, if TACACS is configured properly and it reported auth_err, then don't try fail through to traditional unix authentication through /etc/passwd.

If this failthrough is intended, make it explicit through "sudo config aaa authentication failthrough enable"

Removed an unused variable "aaa.fallback"

Tested manually. Note the presence of 'auth_err=die' in all cases except when failthrough is explicitly enabled.

admin@str-s6000-acs-13:~$ sudo config aaa authentication failthrough default; date
Wed Apr  3 23:05:18 UTC 2019
admin@str-s6000-acs-13:~$ ls -lrt /etc/pam.d/common-auth-sonic ; grep 123 /etc/pam.d/common-auth-sonic
-rw-r--r-- 1 root root 1316 Apr  3 23:05 /etc/pam.d/common-auth-sonic
auth    [success=done new_authtok_reqd=done default=ignore auth_err=die]        pam_tacplus.so server=100.127.20.22:49 secret=testing123 login=login timeout=5 try_first_pass
auth    [success=done new_authtok_reqd=done default=ignore auth_err=die]        pam_tacplus.so server=100.127.20.21:49 secret=testing123 login=login timeout=5 try_first_pass

admin@str-s6000-acs-13:~$ sudo config aaa authentication failthrough enable; date ; h4 "AAA|authentication"
Wed Apr  3 23:06:37 UTC 2019
admin@str-s6000-acs-13:~$ ls -lrt /etc/pam.d/common-auth-sonic ; grep 123 /etc/pam.d/common-auth-sonic
-rw-r--r-- 1 root root 1294 Apr  3 23:06 /etc/pam.d/common-auth-sonic
auth    [success=done new_authtok_reqd=done default=ignore]     pam_tacplus.so server=100.127.20.22:49 secret=testing123 login=login timeout=5 try_first_pass
auth    [success=done new_authtok_reqd=done default=ignore]     pam_tacplus.so server=100.127.20.21:49 secret=testing123 login=login timeout=5 try_first_pass

admin@str-s6000-acs-13:~$ sudo config aaa authentication failthrough disable; date ; h4 "AAA|authentication"
Wed Apr  3 23:07:09 UTC 2019
admin@str-s6000-acs-13:~$ ls -lrt /etc/pam.d/common-auth-sonic ; grep 123 /etc/pam.d/common-auth-sonic
-rw-r--r-- 1 root root 1321 Apr  3 23:07 /etc/pam.d/common-auth-sonic
auth    [success=done new_authtok_reqd=done default=ignore auth_err=die]        pam_tacplus.so server=100.127.20.22:49 secret=testing123 login=login timeout=5 try_first_pass
auth    [success=done new_authtok_reqd=done default=ignore auth_err=die]        pam_tacplus.so server=100.127.20.21:49 secret=testing123 login=login timeout=5 try_first_pass

files/image_config/hostcfgd/hostcfgd

@@ -44,8 +44,6 @@ class AaaCfg(object):
     def __init__(self):
         self.auth_default = {
             'login': 'local',
-            'failthrough': True,
-            'fallback': True
         }
         self.tacplus_global_default = {
             'auth_type': TACPLUS_SERVER_AUTH_TYPE_DEFAULT,