Allow to configure the libguestfs cache and tmp dirs

[david-caro]

From the #94:

That's a good idea - I use it without any issues for some time now. Two more enhancements: export LIBGUESTFS_CACHEDIR=/home/zram export LIBGUESTFS_TMPDIR=/home/zram

[nvgoldin]

Testing I have done with those parameters didn't yield major improvements. I think bootstrap stage is pretty fast. Also, as we use scp in collect now, bootstrap/customize is the only gain.

Second, you can always run export LIBGUESTFS... prior to running Lago and it will use them.