Remove hard-coded valid eauth backends

We can't easily guess possible valid values here since new backend
modules can be easily added outside from the modules that ship with the
core Salt distribution. That said, if the user doesn't provide a value
here optparse will just grab anything the comes next, even another valid
flag (is that right?) so it would be nice to have some kind of
validation here...

scripts/pepper

@@ -131,7 +131,6 @@ def get_login_details(opts):
         'SALTAPI_EAUTH': 'auto',
     }
 
-    valid_auths = ('pam')
     profile = 'main'
 
     config = ConfigParser.RawConfigParser()
@@ -148,7 +147,7 @@ def get_login_details(opts):
         results[key] = os.environ.get(key, results[key])
 
     # get eauth prompt options
-    if opts.eauth and opts.eauth in valid_auths:
+    if opts.eauth:
         results['SALTAPI_USER'] = raw_input('Username: ')
         results['SALTAPI_PASS'] = getpass.getpass(prompt='Password: ')