Use Slate's `slateConfig.cmake` instead of `FindSlate.cmake`

[v-dobrev]

Follow up from #48 to consider after the v0.4.0 release.

After some discussions in #46, we should consider updating this PR to use slateConfig.cmake instead of the cmake/FindSlate.cmake module. If we do that, there will be no need to explicitly link the PLASMA example with OpenMP -- that dependency will come from slateConfig.cmake.

Any thoughts?

Originally posted by @v-dobrev in #48 (comment)

Also consider this suggestion: #48 (comment)