Changes

testenv: don't use all RAM in 5gc testsuite build

When building the 5gc testsuite with as many parallel jobs as CPU cores
are available (the default), on at least Pau's and my system, all RAM
gets consumed and the system becomes unusable. On other systems, an
out-of-memory killer might kill the compiler, which is also not great.
This can currently only be avoided if the user remembers to pass "-j4"
to testenv, to ensure that only 4 jobs run in parallel.

Add a new max_jobs_per_gb_ram= option to 5gc/testenv.cfg. Set it to 0.3,
so on e.g. systems with 15 GiB of RAM, we get 4 jobs as maximum.

Change-Id: I1b9c9a2e3fa244337342e95457ac6d3a8caa0ae6