Avoid creation of tap interfaces during system check.

Running "tunctl" will result in creation of a tap interface. In order to avoid
it call it with some "-d" parameter which will make it exit fast.

slapos/format.py

@@ -1040,8 +1040,10 @@ class Config(object):
   def checkRequiredBinary(binary_list):
     missing_binary_list = []
     for b in binary_list:
+      if type(b) != type([]):
+        b = [b]
       try:
-        callAndRead([b])
+        callAndRead(b)
       except ValueError:
         pass
       except OSError:
@@ -1126,7 +1128,7 @@ class Config(object):
       if self.alter_user:
         self.checkRequiredBinary(['groupadd', 'useradd', 'usermod'])
       if self.create_tap:
-        self.checkRequiredBinary(['tunctl'])
+        self.checkRequiredBinary([['tunctl', '-d']])
       if self.alter_network:
         self.checkRequiredBinary(['ip'])
     # Required, even for dry run