erp5_web: Fix StaticWebSection support for VirtualHostMonster.

There are 2 shemes for VIRTUAL_URL_PARTS:
- 2-tuple with SERVER_URL and subpath
- 3-tuple with SERVER_URL, vh path and subpath
This code only supported the 2-tuple version, add support for the 3-tuple
version.
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passed SlapOS.Eggs.UnitTest-Master.Python2

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failed SlapOS.Eggs.UnitTest-Master.Python3

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