segbot_bringup: Teleop twist keyboard node has unsafe speeds

[maxsvetlik]

The default linear and angular speed should be lower. There is also a bug in the node that repeats the last velocity command, when you adjust the speed limits.

To reproduce this:

• bring up the robot stack, run the teleop_twist_keyboard node.
• Move the robot forward with 'i'
• Slow the speed limits with 'z'