lxd: Set default expiry for backups
lxd: Set default expiry for backups

A default expiry of 30 minutes is set if none is provided.

Signed-off-by: Thomas Hipp <5f82c492b3b00e427412d216ce820707a10c51ce@canonical.com>

hallyn/lxd,mjeanson/lxd,stgraber/lxd,stgraber/lxd,lxc/lxd,Skarlso/lxd,atxwebs/lxd,stgraber/lxd,Skarlso/lxd,lxc/lxd,lxc/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,mjeanson/lxd,Skarlso/lxd,stgraber/lxd,hallyn/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,stgraber/lxd,atxwebs/lxd,mjeanson/lxd,atxwebs/lxd,hallyn/lxd,lxc/lxd,atxwebs/lxd,Skarlso/lxd,lxc/lxd,hallyn/lxd