test: create asok files in a temp directory under $TMPDIR

test: create asok files in a temp directory under $TMPDIR

to shorten the pathname of unix domain socket created for admin socket,
so it does not exceed the limit of 107 on GNU/Linux:

* ceph-helper.sh: the temp directory is named ${TMPDIR:-/tmp}/ceph-asok.$$
* vstart.sh: the temp directory is named `mktemp -u -d "${TMPDIR:-/tmp}/ceph-asok.XXXXXX"`

Fixes: http://tracker.ceph.com/issues/16895
Signed-off-by: Kefu Chai <kchai@redhat.com>