Replace /tmp/cephtest/ with configurable path

Replace /tmp/cephtest/ with configurable path

Teuthology uses /tmp/cephtest/ as the scratch test directory for
a run.  This patch replaces /tmp/cephtest/ everywhere with a
per-run directory: {basedir}/{rundir} where {basedir} is a directory
configured in .teuthology.yaml (/tmp/cephtest if not specified),
and {rundir} is the name of the run, as given in --name.  If no name
is specified, {user}-{timestamp} is used.

To get the old behavior (/tmp/cephtest), set test_path: /tmp/cephtest
in .teuthology.yaml.

This change was modivated by #3782, which requires a test dir that
survives across reboots, but also resolves #3767.

Signed-off-by: Sam Lang <sam.lang@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>