git: Allow adding command to cgroup to fail

Cgroups offer protection at the OS level from certain Git commands taking up too many system resources. However, there are situations where cgroups can suddenly become unavailable, such as on a restart of a server. In these situations, Gitaly should not be prevented from starting up, since Cgroups is not critical to the correct functioning of Gitaly. Instead, we should enter into a degraded state whereby Gitaly can still function, just not with the protection Cgroups offers. This change allows Gitaly to start up without Cgroups, as well as execute commands even if it fails getting added to a cgroup.

fixes: #3962 (closed)