Fix #45, Refactor code to fit suggested app model

[skliper]

Describe the contribution
This is #58, resubmitted for fixes:
Addresses ticket #45. Specifically adds new sample_app_cmds.c/.h and sample_app_utils.c/.h files, moves functions out of sample_app.c into the new cmds and utils files. Updates unit tests.
This change makes sample_app a better reference application - structure more closely matches other apps.

Fix #45

Testing performed
TBD

Expected behavior changes
None

System(s) tested on

• Hardware: [e.g. PC, SP0, MCP750]
• OS: [e.g. Ubuntu 18.04, RTEMS 4.11, VxWorks 6.9]
• Versions: [e.g. cFE 6.6, OSAL 4.2, PSP 1.3 for mcp750, any related apps or tools]

Additional context
None

Third party code
None

Contributor Info - All information REQUIRED for consideration of pull request
Jacob Hageman - NASA/GSFC (resubmit)
Elizabeth Timmons/NASA (implemented)

[skliper]

Currently has CI errors: https://travis-ci.com/github/skliper/cFS/jobs/333841039

@ejtimmon - Do you have time to address these?