Do not overprovision disk space by default

Added

• Configuration option of overprovisioning the build volume (overprovision-lvm), defaulting to 'false'.
• Configurable temp volume reserve (temp-reserve-mb), defaulting to '100'.

Changed

• (potentially breaking) The LVM image files are not created sparsely anymore. Previously, free disk space appeared free on both the build volume and the hosting volume, until actually allocated by writing to the build volume. Now, the space is actually consumed on volume creation -- meaning, that after creating the build volume, the root and temp volume do not have or show more space available than configured in root-reserve-mb and temp-reserve-mb. This can be reverted by setting overprovision-lvm to 'true', but surprising out-of-disk space situations might be the result.
• Temp volume reserve was fixed to 1024 KB, this defaults to a more cautious 100 MB now, and is configurable.