Rolling Pool Update: check/handle HA/load balancing

[olivierlambert]

As Citrix doc said, we MUST disable HA before doing RPU.

Before starting an upgrade, the wizard conducts a series of prechecks to ensure that certain pool-wide features, such as HA and WLB, are temporarily disabled and that each host in the pool is prepared for upgrade (for example, that the CD/DVD drive of each host is empty)

https://docs.citrix.com/en-us/xencenter/7-1/upgrade.html

In our case, we should add that:

1. In the modal asking for confirmation, if we detect HA, we should add a sentence that HA will be de-activated until the RPU is done
2. If we confirm to make the RPU, then XO should disable HA before doing updates and reboot/evacuate
3. When the RPU is finished, we should re-enable HA

I don't know if we already check about removing CD drives (but we probably should)

Lastly, we might have to disable our load balancing plugin during the operation (and re-enable it after).