Support for service cache updates

[niusmallnan]

When we get services from os-services via ros service, ROS can download the files to cache, they will be saved to /var/lib/rancher/cache.

These caches are now unable to be updated, We should support updating the cached files.

We can add an option to ros service list, suck as:

$ ros service list --update

Also, need to apply engine and console and os index.

[rootwuj]

Tested with rancher/os:v1.5.2-rc1 from May 22

Run the following command can update the cache file (/var/lib/rancher/cache/), and other files are not affected.

• sudo ros service list --update
• sudo ros console list --update
• sudo ros engine list --update
• sudo ros os list --update

The following command can work with the updated cache file.

• sudo ros service up
• sudo ros console switch
• sudo ros engine switch
• sudo ros os upgrade

After the reboot or shutdown, everything went as expected.

Verified fixed