CA-108379: Hotfix prechecks encourage VMs to be suspended unnecessarily ...

...if HA is enabled - Fixed

- Added three Warning classes (HAEnabledWarning, WLBEnabledWarning, HostNotLiveWarning), used in the Pre-checks page of the Install wizard (also in the Rolling Pool Upgrade wizard) to indicate that a check was skipped and the reason why.
- If HA or WLB is on, we show this as an error in the "HA and WLB status" check and the "VM migration status" checks will be skipped (with a warning displayed) for all the hosts in the pool.
- Similarly, if a host if unreachable, we show this as an error in the "Host liveness status" check and the following checks for that host will be skipped (and warning displayed).

Signed-off-by: Mihaela Stoica <mihaela.stoica@citrix.com>