This partially reverts the "Get rid of ssconf" patch.

It adds back a simpler version of the SimpleStore class, and drops the
WritableSimpleStore class. The new version of the class also has
node_list as a new key, and increases the size of the keys so that big
clusters will fit the node list. Also, the SS_* constants are moved to
constants.py, since the ConfigWriter class will need them too in order
to generate the values dictionary.

It also changes the GetMasterAndMyself function to use the SimpleStore
by default, and the backend._GetConfig to use it too (it has all the
needed keys).

Reviewed-by: imsnah