Handle jsbsim bridge configurations

[Jaeyoung-Lim]

Problem Description There needs a way to be able to configure the jsbsim in various ways.

• Configure the JSBSim FDM models
• Configure sensor configurations on the vehicle

Solution This adds a configuration parser that configures the jsbsim bridge with three systems. The lower elements on the list have higher priorities.

• Configuration files: There exists one configuration file for each models. The configuration file exists under the config directory.
• Command line arguments: Command line arguments are passed to configure the bridge dynamically on execution.
• Environment variables: Use environment variables to override some configurations.

This also improves what are passed as command line arguments. The improvements are as the following.

• The command line arguments that are necessary is reduced to 1. Although it sitll uses the 3rd argument, once this PR is merged I will switch to getopt based command line interface, since this also requires changes in the sitl_run.sh script in PX4/Firmware
• The model directory/model file that was being handled in https://github.com/PX4/Firmware/blob/master/Tools/setup_jsbsim.bash is no longer necessary. Instead, it is specified as part of the config file. If the jsbsimbridge tag is not specified, it defaults to the models/<model>/<model>.xml path as this is the default behavior of JSBSim

  <jsbsimbridge>
      <aircraft_directory>models/Rascal</aircraft_directory>
      <aircraft_model>Rascal110-JSBSim</aircraft_model>
  </jsbsimbridge>

Additional Context

• This change is needed to make the scenario loading more convenient from https://github.com/PX4/px4-jsbsim-bridge/pull/7
• Multiple vehicle scenarios are not incorporated in the design yet.

[Jaeyoung-Lim]

@bkueng This is ready for review now!

[Jaeyoung-Lim]

@bkueng Done!

[Jaeyoung-Lim]

@bkueng Thanks!