vstart.sh: specify mon_data_avail_crit in ceph.conf

vstart.sh: specify mon_data_avail_crit in ceph.conf

ceph-mon consumes this option when it boots, and exits if the ratio
of free space is  lower than the specified number, which is 5% by
default. but we use `ceph -c $conf_fn config assimilate-conf -i -`
to absorb these option after monitor starts. so, without this change,
the default value of mon_data_avail_crit is always used, if machine
has lower ratio of free space on the partition where mon store is
located, ceph-mon just exists with the error message like:

2021-05-24T01:53:14.644+0000 7ff64961e580 -1 error: monitor data
filesystem reached concerning levels of available storage space
(available: 4% 17 GiB)

after this change, the option is written in ceph.conf, and can be
read by ceph-mon when it boots. so the overriden value of 1% has
the chance to take effect. this helps to address some test failures
found in our "make check" runs performed by jenkins on machines whose
disk space is enough for completing the test, but its ratio of free
space is lower than 5%.

Signed-off-by: Kefu Chai <kchai@redhat.com>