lxc/instance/drivers/driver/common: Removes empty value check from insertConfigkey
lxc/instance/drivers/driver/common: Removes empty value check from insertConfigkey

We don't know if the user actually wants to store an empty value, and either way if it is there already we cant replace it.

Signed-off-by: Thomas Parrott <6b778ce645fb0e3dde76d79eccad490955b1ae74@canonical.com>

lxc/lxd,atxwebs/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,lxc/lxd,atxwebs/lxd,stgraber/lxd,stgraber/lxd,hallyn/lxd,atxwebs/lxd,stgraber/lxd,hallyn/lxd,atxwebs/lxd,lxc/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd