config: Create ast_variable_find_in_list()

Add
const char *ast_variable_find_in_list(const struct ast_variable *list,
   const char *variable);

ast_variable_find() requires a config category to search whereas
ast_variable_find_in_list() just needs the root list element which is
useful if you don't have a category.

Tested-by: George Joseph

Review: https://reviewboard.asterisk.org/r/4217/



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@428733 f38db490-d61c-443f-a65b-d21fe96a405b