daemon: execdriver: lxc: fix set memory swap

On LXC memory swap was only set to memory_limit*2 even if a value for
memory swap was provided. This patch fix this behavior to be the same
as the native driver and set correct memory swap in the template.
Also add a test specifically for LXC but w/o adding a new test
requirement.

Signed-off-by: Antonio Murdaca <runcom@linux.com>