Changed strategy of cgroup fs management in hv_lxc
Changed strategy of cgroup fs management in hv_lxc

The existing implementation of GetCgroupMountPoint has a problem because
there is no guarantee that the first element of /proc/mounts which has
fstype == cgroup contains all cgroup subsystems.
Changes made by this commit are:
- Introduce the _MountCgroupSubsystem method to mount the cgroup
  subsystem fs manually.
- Introduce the _GetOrPrepareCgroupSubsysMountPoint method to check if
  the cgroup subsystem is already mounted, and invoke
  _MountCgroupSubsystem if not. Return the path of the mounted
  subsystem.
- Replace invokation of GetCgroupMountPoint by
  _GetOrPrepareCgroupSubsysMountPoint.

Signed-off-by: Yuto KAWAMURA(kawamuray) <12589a13a27e6c6eeb0b2b8963a6f3e92691a9e2@gmail.com>
Reviewed-by: Hrvoje Ribicic <be5828fca31f89218048bec4189e30a581aa65da@google.com>

onponomarev/ganeti,bitemyapp/ganeti,yiannist/ganeti,ganeti/ganeti,yiannist/ganeti,andir/ganeti,mbakke/ganeti,ganeti/ganeti,apyrgio/ganeti,mbakke/ganeti,ganeti-github-testing/ganeti-test-1,andir/ganeti,bitemyapp/ganeti,ganeti-github-testing/ganeti-test-1,leshchevds/ganeti,apyrgio/ganeti,andir/ganeti,grnet/snf-ganeti,leshchevds/ganeti,dimara/ganeti,leshchevds/ganeti,ganeti/ganeti,yiannist/ganeti,mbakke/ganeti,dimara/ganeti,onponomarev/ganeti,grnet/snf-ganeti