Add wordexp_ros wrapper for wordexp, allowing ROS-aware path expansion

[AAlon]

wordexp (see http://man7.org/linux/man-pages/man3/wordexp.3.html) expands tilde by looking at $HOME. If $HOME is unset it will get the home directory from the pwd entry (pw_dir returned from http://man7.org/linux/man-pages/man3/getpwnam.3.html).
For ROS applications, before falling back to the pwd entry we'd like to use ROS_HOME if it exists.

This would result in a functionality similar to the one implemented at aws-robotics/cloudwatch-common#43, but better because we'd fallback to the pwd entry if no env variables are set. Plus, putting it here in aws_common so it'll be usable by other CEs, specifically for the rosbag uploader.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #52 into master will increase coverage by 0.62%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #52      +/-   ##
==========================================
+ Coverage   89.03%   89.65%   +0.62%     
==========================================
  Files          16       18       +2     
  Lines         830      880      +50     
==========================================
+ Hits          739      789      +50     
  Misses         91       91

Flag	Coverage Δ
#ROS_1	89.65% <100%> (+0.62%)	⬆️
#ROS_2	91.83% <100%> (+0.5%)	⬆️
#dashing	91.83% <100%> (+0.5%)	⬆️
#kinetic	89.75% <100%> (+0.62%)	⬆️
#melodic	91.83% <100%> (+0.5%)	⬆️

Impacted Files	Coverage Δ
aws_common/test/fs_utils/wordexp_ros_test.cpp	100% <100%> (ø)
aws_common/src/fs_utils/wordexp_ros.cpp	100% <100%> (ø)

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[ryanewel]

Nice!