Add move base flex

[FlorGrosso]

Description

Adds move base flex as a navigation alternative to move base.

Type of change

Please delete options that are not relevant.

• [x] New feature (non-breaking change which adds functionality)
• [x] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] This change requires a documentation update

How Has This Been Tested?

A) Regressions Check that move base is the navigation option by default

export LOCALIZATION=amcl
export RVIZ=true
export LASER=rplidar
roslaunch ca_gazebo create_house.launch

B) New feature

export LOCALIZATION=amcl
export RVIZ=true
export LASER=rplidar
export NAVIGATION=mbf
roslaunch ca_gazebo create_house.launch

Using rviz, test that the robot can navigate when sending a goal to create1/move_base_simple/goal.

Note: test using different local and global planners, configuring them through env variables.

Test Configuration:

• [x] Gazebo simulation

Checklist:

• [x] My code follows the style guidelines of this project (Travis CI is passing)
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings
• [x] I have added tests that prove my fix is effective or that my feature works
• [x] New and existing unit tests pass locally with my changes

[FlorGrosso]

@eborghi10 I've just updated this PR to complete the dynamic config of local and global planners using env variables. This is ready for review now.

[FlorGrosso]

@eborghi10 all comments addressed. Could you please take another look? thanks!