Fix incorrect conversion of string to list in ConfigLoader

Casting a string to a list in Python (via `list('hello')`) creates a
list consisting of each character in the string.

We had code in BaseConfigLoader to convert a conf value into a list
when a list was expected, but the conversion was incorrect. The
expected behavior is that a single-element list is created containing
the original value (e.g., ['hello'], not ['h', 'e', 'l', 'l', 'o']).
Merge pull request #20 from box/fix_issue_with_conf_slave_list

Fix incorrect conversion of string to list in ConfigLoader