ipatests: Invoke class install methods properly with respect to pytest-multihost

Multihost object was is not passed to the install method in the super construction.
This fixes setup errors in AD Trust, Forced client reenrollment, CALess and Sudo
tests.

https://fedorahosted.org/freeipa/ticket/4809
ipatests: Set the correct number of required clients for IntegrationTest

Remove the copy-pate error that used number of required replicas instead.

https://fedorahosted.org/freeipa/ticket/4809
ipatests: Refactor and fix docstrings in integration pytest plugin

https://fedorahosted.org/freeipa/ticket/4809