Simplify OpenVPN and virtual plugin auto-install checks

[Salvialf]

• network::dns_create() (OpenVPN) and jeeObject::createSummaryToVirtual() (virtual) both had duplicated install-retry logic between a try block and its catch, with a dead if (!is_object($plugin)) branch that could never trigger since plugin::byId() never returns a non-object without throwing. Both now use plugin::isInstalled() to decide whether an install is needed, removing the duplication and the unreachable branch.
• As a side effect, this also restores reachability of the intended error message when the second plugin::byId() attempt fails after a failed install (previously, an uncaught exception from that second call bypassed the friendly "plugin must be installed" message).
• createSummaryToVirtual()'s sleep(2) after doUpdate() is commented out rather than removed: it dates back to a 2016 fix with no documented root cause, doUpdate() is fully synchronous today, and unlike OpenVPN this flow never calls dependancy_install(). Kept as a comment pending real-world confirmation it's safe to drop entirely.