Spawn rosbridge in new shell to connect to a roscore

[AWatk]

From @dwatkins on November 2, 2017 17:4

Spawn a worker process to contain rosbridge and roslib interfaces for each roscore.

Rosbridge needs to run in a new shell for each roscore.

Needs to:

1. source message definitions for all packages on the system
2. generate a unique port to reside on

Copied from original issue: rosmod/rosmct#2

[AWatk]

only spawn a new shell for rosbridge to reside at.

[AWatk]

updated to be on rosmod's end. no long need to deal with roslib interaction, which is handled in rosmct

[AWatk]

related to #182, #184, #185, #187

[finger563]

Need to figure out how to properly manage rosbridge_server's lifecycle (so we can kill it when we StopExperiment)

[AWatk]

ocal host also needs to export ROS_IP to the ip address reachable by hosts in the system. Need for roscore to arbitrate pub/sub advertisements correctly

[finger563]

Will have to update the plugin so that the plugin can know by what IP the ROSMOD system can reach the rosmod server (esp. since the server can and usually does have more than one set of IP addresses)