add -f option to lxc-attach
add -f option to lxc-attach

Otherwise a container with a non-standard configuration file
can be started but not attached to.
Fixes the following case:
 # lxc-start -n ct -f /different/path/my.config
 # lxc-attach -n ct
 Error: container ct is not defined

Signed-off-by: Wolfgang Bumiller <bac7898f539fd233d9ce33be7a9eda13ee647930@proxmox.com>

thmo/lxc,ysbnim/lxc,ss1h2a3tw/lxc,cjwatson/lxc,ss1h2a3tw/lxc,hallyn/lxc,hallyn/lxc,thmo/lxc,federicobriata/lxc,cjwatson/lxc,federicobriata/lxc,GreatFruitOmsk/lxc,thmo/lxc,ss1h2a3tw/lxc,LynxChaus/lxc,aeris/lxc,aeris/lxc,caglar10ur/lxc,hfuCN/lxc,caglar10ur/lxc,GreatFruitOmsk/lxc,ysbnim/lxc,hallyn/lxc,terceiro/lxc,hfuCN/lxc,federicobriata/lxc,LynxChaus/lxc,hfuCN/lxc,LynxChaus/lxc,aeris/lxc,GreatFruitOmsk/lxc,terceiro/lxc,caglar10ur/lxc,cjwatson/lxc,ysbnim/lxc,terceiro/lxc