Reload SCS binary when updating security-containers

Reload SCS binary when updating security-containers

[Feature]       Reload SCS without turning containers off when binary is updated.
[Cause]         When updating SCS we don't want to restart containers, only SCS itself.
[Solution]      Add SIGUSR1 handling which will tell SCS to keep containers alive when exiting.
                Add check in ContainerConnectionTransport if containers are running to skip remount
                of tmpfs when it is not needed.
[Verification]  Build, install, reboot target. Test the following when SCS is running together with
                containers active:
                  * Call "systemctl stop security-containers". SCS should turn off and containers
                    should turn off as well. Call "systemctl start security-containers", SCS and
                    containers should start up.
                  * Simulate update by calling "kill -USR1 `pidof security-containers-server`". SCS
                    should properly reload, however containers should stay on. (note - the best way
                    to check it would be by verifying logs in journalctl).

Change-Id: I3a6d0fb25a4579208ad0f6d0de00e2755548230e
Signed-off-by: Lukasz Kostyra <l.kostyra@samsung.com>