change host into string

[knorth55]

I got following error in subprocess in 0.5.1 melodic. This PR prevent this error just changing host to str(host)

[ERROR] [1566054843.135049]: Exception in your execute callback: execv() arg 2 must contain only strings
Traceback (most recent call last):
  File "/opt/ros/melodic/lib/python2.7/dist-packages/actionlib/simple_action_server.py", line 289, in executeLoop
    self.execute_callback(goal)
  File "/opt/ros/melodic/lib/mongodb_store/replicator_node.py", line 110, in move_entries
    self.do_restore(extras, db=goal.database)
  File "/opt/ros/melodic/lib/mongodb_store/replicator_node.py", line 135, in do_restore
    self.restore_process = subprocess.Popen(rest_args)
  File "/usr/lib/python2.7/subprocess.py", line 394, in __init__
    errread, errwrite)
  File "/usr/lib/python2.7/subprocess.py", line 1047, in _execute_child
    raise child_exception
TypeError: execv() arg 2 must contain only strings

$ dpkg -l | grep mongodb-store
ii  ros-melodic-mongodb-store                       0.5.1-2bionic.20190628.135820                amd64        A package to support MongoDB-based storage and analysis for data from a ROS system, eg.
ii  ros-melodic-mongodb-store-msgs                  0.5.1-2bionic.20190628.135414                amd64        The mongodb_store_msgs package

[hawesie]

THanks!