slapgrid: do NOT raise in case of servererror when calling /softwareAvailable.

Mostly informational, it is more improtant to have a functional slapgrid.
Also: remove trailing spaces.
slapproxy tests: start external proxy using local ipv4 if provided in env.
slapproxy test: use current running code to start external proxy.
slapgrid: mandatory parameters can NOT be empty.
slap test: make sure mocks/stubs are NOT leaking.