Tentacle uninstall in OnStop and get the latest Tentacle installer

This fixes a problem where roles will sometimes get into a corrupted state after normal restarts for patching where the existing tentacle install is corrupted.

By uninstalling the tentacles in OnStop and cleaning all trace of them it means that if the same VM is then used for OnStart again afterwards it should work fine.

This change also changes it to install the latest version of OctopusDeploy Tentacle rather than a hard-coded one thanks to a new latest link added by the OctopusDeploy guys (thanks @paulstovell / @nblumhardt).