Run Security Containers Server as non root user

Run Security Containers Server as non root user

[Bug/Feature]  Drop root privileges of the server during startup.
[Solution]     * User "security-containers" has been added to the
                 "libvirt" group.
               * CAP_SYS_ADMIN and CAP_MAC_OVERRIDE capabilities have
                 been provided using libcap-ng.
[Verification] 1. Make sure that "security-containers" user (with UID
                  == 377) exists in the conainers. If no, execute:
                      chroot /path/to/container /bin/bash \
                      -c "useradd -r security-containers -u 377"
               2. Run tests.
               3. Start SCS service as root (directly or via systemd
                  service). Verify /proc/<PID>/status of the process:
                   * Uid == 377
                   * CapPrm == CapEff == 0000000000200000
                   * Groups: <libvirt group ID>, <input group>
               4. Run the service with "--root" option. Remember to
                  change policy in dbus configuration file
                  "etc/dbus-1/system.d/com.samsung.containers.conf"
                  from "security-containers" to "root".
               5. Trigger update (via sending SIGUSR1) and check if
                  UID, groups and capabilities set did not change.

               NOTE: Latest libvirt (from "tizen" branch on
               tizen.org) is required.

Change-Id: Idfda05fb081ca48193b19a99a6628cf14ec4bf57
Signed-off-by: Michal Witanowski <m.witanowski@samsung.com>