[r=wallyworld],[bug=1318485] Use lxc clone by default
All providers now support configuring lxc clone support, not just local provider. The config attributes which previously were just for local provider now are available on all providers: lxc-clone lxc-clone-aufs
lxc-clone, if unspecified, will be set to true if the underlying platform supports it. The test is for Ubuntu trusty or greater. The final check is done when the container manager is created.
1.19.2 used a setting lxc-use-clone. This is now removed in place of the universal lxc-clone setting.