Tag container tests that use cGroups with cgroups keyword

Some container tests use cGroups, while other test do not. This RFE is to tag such tests with "@key cgroups".

With advent of PodMan and cGroups v2 a specific environment and configuration is required, and it is important to know which tests use cGroups.

For instance, with RedHat 8.1 and Oracle Linux 8.1 cGroups will not work with 'rootless' containers. cGroups v2 does not come out-of-box and requires special setup.