vitastor-disk prepare: implement first form of the command

src/str_util.h

@@ -8,6 +8,8 @@
 std::string base64_encode(const std::string &in);
 std::string base64_decode(const std::string &in);
 uint64_t parse_size(std::string size_str);
+std::string strtolower(const std::string & in);
+std::string trim(const std::string & in);
 uint64_t stoull_full(const std::string & str, int base = 0);
 std::string format_size(uint64_t size, bool nobytes = false);
 void print_help(const char *help_text, std::string exe_name, std::string cmd, bool all);