Merge branch Feature/auto-vic-ui-plugin (#1979) (#1998)

We switch ui plugin registration from running manual commands of install.sh or upgrade.sh to automation. A set of rest apis are hosted on the VIC appliance fileserver. During the appliance initialization, index.html page will send a post request to the plugin install api, which will prepare the plugin and register the plugin to vCenter.

A thumbprint verification page is shown to enhance security, e.g. a wrong vCenter or a man-in-the-middle attack.

In case there is an old ui plugin in vCenter already, the new VIC appliance will overwrite the old ui plugin and register a new one.

UI plugin registration is also integrated into upgrade.sh, so the UI plugin can be upgraded as well when running upgrade.sh for VIC appliance upgrade.

With UI plugin auto registration, we do not need the manual ui install/upgrade.sh scripts and vic-ui-* commands anymore.

UI plugin registration is set to be installed by default during initialization and upgrade, but users can also disable the installation by uncheck the installation option.

(cherry picked from commit 0220fd5)