_artifactcache.py: Don't require the quota to be available on disk.

Instead only rely on the headroom to be enough to protect against
out of space conditions. The headroom can become configurable as
a separate step is required.

The changes to achieve this are:

  * Rename ArtifactCache.has_quota_exceeded() to ArtifactCache.full().

  * ArtifactCache.full() now also reports True if the available
    space on the artifact cache volume is smaller than the headroom.

    This ensures jobs get triggered to cleanup the cache when
    reaching the end of the disk.

  * When loading the artifact quota, it is now only an error if
    the quota exceeds the overall disk space, not if it does not
    fit in the available space.

    It is still a warning if the quota does not fit in the
    available space on the artifact cache volume.

  * Updated scheduler.py and buildqueue.py for the API rename

  * tests: Updated the artifactcache/expiry.py test for its
    expectations in this regard.

    Added a new test to test an error when quota was specified to
    exceed total disk space, and adjusted the existing tests to
    expect a warning when the quota does not fit in the available
    space.

This fixes issue #733 and #869.