orchestra/remote.py: use ensure_online() helper in run()

orchestra/remote.py: use ensure_online() helper in run()

for better readability, and to have better errors when we fail to
connect to remote. before this change, we could have following
backtrace:

2020-03-25T09:00:47.022 INFO:teuthology.lock.ops:Checking smithi088.front.sepia.ceph.com
2020-03-25T09:00:47.023 INFO:teuthology.lock.ops:New key found. Updating...
2020-03-25T09:00:47.044 INFO:teuthology.lock.ops:Updating [smithi088.front.sepia.ceph.com]: set os type and version on server
2020-03-25T09:00:47.044 INFO:teuthology.orchestra.remote:Trying to reconnect to host
2020-03-25T09:00:47.045 DEBUG:teuthology.orchestra.connection:{'username': 'ubuntu', 'hostname': 'smithi088.front.sepia.ceph.com', 'timeout': 60}
2020-03-25T09:00:47.046 DEBUG:teuthology.orchestra.remote:[Errno None] Unable to connect to port 22 on 172.21.15.88
2020-03-25T09:00:47.046 ERROR:teuthology.run_tasks:Saw exception from tasks.
Traceback (most recent call last):
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/run_tasks.py", line 89, in run_tasks
    manager.__enter__()
  File "/usr/lib/python2.7/contextlib.py", line 17, in __enter__
    return self.gen.next()
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/task/internal/lock_machines.py", line 78, in lock_machines
    os_version, arch)
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/lock/ops.py", line 147, in lock_many
    update_nodes(reimaged)
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/lock/ops.py", line 33, in update_nodes
    inventory_info = remote.inventory_info
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/orchestra/remote.py", line 475, in inventory_info
    node['arch'] = self.arch
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/orchestra/remote.py", line 459, in arch
    self._arch = self.sh('uname -m').strip()
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/orchestra/remote.py", line 242, in sh
    proc=self.run(**kwargs)
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/orchestra/remote.py", line 198, in run
    r = self._runner(client=self.ssh, name=self.shortname, **kwargs)
  File "/home/teuthworker/src/git.ceph.com_git_teuthology_master/teuthology/orchestra/run.py", line 412, in run
    transport = client.get_transport()
AttributeError: 'NoneType' object has no attribute 'get_transport'

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