Initial version of main

This includes:
- printing of task list in help output.
- some basic env settings from CLI args.
- extraction of task arguments.
- execution of fabric task.

Change-Id: I801366a5efe18608eb52f740d4f7b622782bc409
Enforce required arguments

Change-Id: Iba37da4cf7c63dfe5543ad8c232ccd9d9799d8ad
Merge "Initial version of main"
Merge "Enforce required arguments"