updateConfiguration is a method to set the list of partitions, not to
add partitions to an existing computer. The previous (slapos.core
<1.5.0) implementation of slap proxy was incorrect and was adding more
partitions each time it was called, but that was never the indented
behavior.

Prepare the list of partitions and call updateConfiguration only once to
set all partitions.

This should fix SLAPOS-SR-TEST that are still using erp5.util

erp5.util.testbrowser: Require zope.testbrowser >= 5.0.0 since 5a0b3763 so update dependency accordingly.

- requests to bootstrap and site availability urls using a script run with userhosts
- hardcoded parameters removed
- try-exception controls in communication with master
- general refactoring and cleanup

This runner is used by scalability test, it communicates
with ERP5 Master and launch test suite (using performance_tester).