Fix warnings from go vet

Signed-off-by: Alexandr Morozov <lk4d4math@gmail.com>
Merge pull request #156 from LK4D4/fix_vet_errors

Fix warnings from go vet
Enter cgroups while entering namespaces as part of NsEnter.
Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Enter cgroups as part of nsenter while using systemd.
Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Store all the cgroups paths as part of the state. This simplifies entering cgroups and will be useful for
cleanups too in the future.
Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)
Update cgroups paths in state to be a map with cgroup type as key and path as value.
Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)