luci-app-ddns: improve performance

Every request directed to the ddns app call ddns tools module.
Ddns tools module have lots of global variable that call slow os.execute function. This adds 10 second to every ddns request even if the function that is requested doesn't need that global variable. This commit introduce env_info function that execute os.execute command by executing what is actually requested and not process all the variables. Also remove 2 unecessary module that are not used. More researh find that major slowdown was caused by the calling of ddns script for the version check. Now we check if opkg is present and use it to check ddns-scripts version.

Signed-off-by: Ansuel Smith <ansuelsmth@gmail.com>