Avoid to use StringUtils::sprintf() with 2 args.

Because wrong signature of one is used on my environment:

 Program received signal SIGSEGV, Segmentation fault.
 0xb7b776a5 in vfprintf () from /lib/i386-linux-gnu/libc.so.6
 (gdb) where
 #0  0xb7b776a5 in vfprintf () from /lib/i386-linux-gnu/libc.so.6
 #1  0xb7c33238 in __vsnprintf_chk () from /lib/i386-linux-gnu/libc.so.6
 #2  0xb7b1dff1 in vsnprintf (__ap=0xb7f82d53 "/usr/local",
     __fmt=0xb7f82d5e "%s/sbin", __n=1024, __s=0xbfffec9c "")
     at /usr/include/i386-linux-gnu/bits/stdio2.h:78
 #3  mlpl::StringUtils::sprintf (fmt=fmt@entry=0xb7f82d5e "%s/sbin",
     ap=ap@entry=0xb7f82d53 "/usr/local") at StringUtils.cc:124
 #4  0xb7ed755a in ConfigManager::reset () at ConfigManager.cc:72
 #5  0xb7f19b80 in reset () at Hatohol.cc:81
 #6  hatoholInit (arg=0xbffff124, arg@entry=0x0) at Hatohol.cc:101
 #7  0x0804a11f in main (argc=3, argv=0xbffff234) at mkTestDB.cc:52