Renamed slaves/frameworks activated/deactivated

The existing terminology is confusing both for "slaves.deactivated" and
"frameworks.activated". Currently a deactivated slave actually
represents a removed/shutdown slave and "frameworks.activated" map holds
both activated and deactivated frameworks.
In order to make things look clear, rename the following:
* master.slaves.deactivated -> master.slaves.removed
* master.slaves.activated -> master.slaves.registered
* master.frameworks.activated -> master.frameworks.registered
* allocator.slaveDisconnect -> allocator.slaveDeactivate
* allocator.slaveReconnected -> allocator.slaveReactivated

Review: https://reviews.apache.org/r/23147
Re-enabled health check tests

Currently the health check tests are disabled as it was flaky. Part of the reason is that all assertions was EXPECTs instead of ASSERTs so it will continue to execute.
Another issue was it's currently discovering the path to launch health check executable from the command executor main argv[0] path.
However, the correct path is to launch the generated script that does library path resolution, which is the parent of .libs.

Review: https://reviews.apache.org/r/23443