qa/standalone/misc: make less noise for asok calculation

qa/standalone/misc: make less noise for asok calculation

The $() notation not only calls the function, it also creates subshell,
which is a separate process. Additionally in debug mode all the content
of the function get_asok_path is printed out in the logs each time
whenever it is called, for example:

    ...stderr://home/ubuntu/cephtest/clone.client.0/qa/standalone/ceph-helpers.sh:552: run_mgr:  get_asok_path
    ...stderr://home/ubuntu/cephtest/clone.client.0/qa/standalone/ceph-helpers.sh:107: get_asok_path:  local name=
    ...stderr://home/ubuntu/cephtest/clone.client.0/qa/standalone/ceph-helpers.sh:108: get_asok_path:  '[' -n '' ']'
    ...stderr:///home/ubuntu/cephtest/clone.client.0/qa/standalone/ceph-helpers.sh:111: get_asok_path:  get_asok_dir
    ...stderr:///home/ubuntu/cephtest/clone.client.0/qa/standalone/ceph-helpers.sh:99: get_asok_dir:  '[' -n '' ']'
    ...stderr:///home/ubuntu/cephtest/clone.client.0/qa/standalone/ceph-helpers.sh:102: get_asok_dir:  echo /tmp/ceph-asok.48301
    ...stderr://home/ubuntu/cephtest/clone.client.0/qa/standalone/ceph-helpers.sh:111: get_asok_path:  echo '/tmp/ceph-asok.48301/$cluster-$name.asok'

Instead of calling get_asok_path each time we need to define osd.0,
etc. asok file path, we just predefine corresponding variables.
It not only avoids extra resource usage, but also removes a lot of
noise from the logs.

Signed-off-by: Kyr Shatskyy <kyrylo.shatskyy@clyso.com>