auto open/close local media sources about ros-webrtc

[aisch]

default is to open/capture on start and keep open. enable on demand open w/ rosparam open_media_sources:

$ rosparam get /ros_webrtc/
cameras:                                                                 
  webcam: {label: webcam, name: 'id:///dev/video0', publish: true}       
ice_servers:                                                             
- {uri: 'stun:stun.services.mozilla.com:3478'}                           
- {uri: 'stun:stun.l.google.com:19302'}                                  
open_media_sources: false                                                
peer_connection:                                                         
  constraints:                                                           
    optional: {DtlsSrtpKeyAgreement: 'true', googVeryHighBitrate: 'true'}
queue_sizes: {audio: 1000, data: 1000, event: 1000, video: 1000}         
trace: {file: /tmp/ros_webrtc.trace, filter: all}                        

if open_media_sources: false then local media will be opened/closed on demand. so e.g. when there are no peer cxns u see local media src has state: ended e.g.:

$ rosservice call /ros_webrtc/get_host
sdp_constraints: 
  mandatory: []
  optional: 
    - 
      key: DtlsSrtpKeyAgreement
      value: true
    - 
      key: googVeryHighBitrate
      value: true
audio_sources: 
  - 
    kind: audio
    label: ''
    state: ended
    publish: False
    rotation: 0
video_sources: 
  - 
    kind: video
    label: webcam
    state: ended
    publish: True
    rotation: 0
peer_connections: []