cmake: use $CMAKE_BINARY_DIR for default $CEPH_BUILD_VIRTUALENV

cmake: use $CMAKE_BINARY_DIR for default $CEPH_BUILD_VIRTUALENV

to avoid conflict with other `make check` runs in the same host

See-also: http://tracker.ceph.com/issues/36737
Signed-off-by: Kefu Chai <kchai@redhat.com>
(cherry picked from commit a8d6b2170dfa0838dad0d1b7b03fc8cf19e4eeff)

Conflicts:
	- drop the change to mgr/*/run-tox.sh, as they don't exist in
luminous.
        - apply the change to ceph-disk/run-tox.sh
	- apply the change to ceph-helpers.sh: as it set $PATH using
          $TMPDIR, which is set to /tmp if $TMPDIR is empty.