FIX Do one run with MiniBatchKMeans and explicit centers

* this will fix issue #2686
* if n_init is not 1, it will be set to one and a warning will be printed
* added a test case which checks for the warning
* modified other test cases so that they do not cause this warning
FIX MBKMeans w/ explicit centers and n_init>1, part 2

* don't change a user-set attribute in fit
* specify, in the warning, which argument to change