adt-build-lxc: uses unknown option -F to lxc-create

Failed to exec "adt-build-lxc debian sid" on trusty.

Already fixed in upstream (and utopic also).
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=745415

Could you backport above patch?

-- from debian bug report ---

adt-build-lxc attempts to use the -F option for lxc-create if the
lxc root already exists, but lxc-create doesn't support this option (at
least not with the lxc currently in sid):

See tools/adt-build-lxc:71

% adt-build-lxc debian sid
getopt: invalid option -- 'F'
lxc-debian -h|--help -p|--path=<path> [-a|--arch]
[-r|--release=<release>] [-c|--clean]
release: the debian release (e.g. wheezy): defaults to current stable
arch: the container architecture (e.g. amd64): defaults to host arch
lxc_container: container creation template for adt-sid.new failed
lxc_container: Error creating container adt-sid.new

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