Xvfb fix and other minor fixes

[rob-clarke]

Changes in this version:

• Gazebo Xvfb now works in network host mode even if X display number 1 is already active on host. Now Xvfb automatically finds a free display number.
• The custom domain for the docs is now (hopefully) no longer overwritten on each documentation update.
• A number of topics have been added to the MAVROS bridge*:
  • ~/altitude
  • ~/extended_state
  • ~/imu/data
  • ~/local_position/pose_cov
  • ~/local_position/velocity_body
  • ~/local_position/velocity_body_cov
  • ~/local_position/accel
  • ~/local_position/odom
  • Removed: ~/global_position/velocity

*This change was already in master, included here as record of dev merge.

Fixes #138 Fixes StarlingUAS/FenswoodScenario#15