In SlapOS, both users or partitions can requests partitions. With the
multimaster support of slapos proxy, all requests where made as a user,
but in the case of recursive slapos - where a partition from the "outer"
slapos includes an "inner" slapos, it makes sense to forward partitions
requests as the partition in the "outer" slapos - this way when this
partition is destroyed all partitions that might have been requested are
also destroyed.

To request as partition, the multi-master entry must define two extra
keys:
 - computer, that can be optained by $${slap-configuration:computer}
from instance buildout
 - partition, that can be optained by $${slap-configuration:partition}

When these are not set, the request will be made as a user, like it was
the case before.

We also change the test to unset SLAPGRID_INSTANCE_ROOT because this
implementation has side effect - the environment variable is set and
never unset. Without this, test fail when running the full test suite
because a previous test was leaking SLAPGRID_INSTANCE_ROOT. This is
definitely something we'll have to improve later.