reachable" in wsrep_init_startup on MTR tests

The initialization & de-initialization of wsrep plugin is solely
controlled by wsrep_on system variable. Now, as wsrep_on is ON by
default, the plugin gets initialized during server start. However,
there would be a leakage in case wsrep_on is turned off before plugin's
deinitialization (while server is shutting down) takes place, in which
case the deinit routine will not be invoked.

In this particular case, wsrep_on is turned off as no wsrep provider
has been specified & thus the valgrind warnings.

Fixed by introducing a flag which tracks and now controls the
(de|)initialization of the wsrep plugin instead on wsrep_on.
Added necessary asserts.

main.connect                             [ pass ]  16534
valgrind_report                          [ pass ]