Initial CLI Refactor

[theseanything]

This PR refactors the CLI into the following:

  create      Creates a stack or a change set.
  delete      Deletes a stack or a change set.
  describe    Commands for describing attributes of stacks.
  execute     Executes a change set.
  generate    Prints the template.
  init        Commands for initialising Sceptre projects.
  launch      Launch a stack or environment.
  list        Commands for listing attributes of stacks.
  set-policy  Sets stack policy.
  status      Print status of stack or environment.
  update      Update a stack.
  validate    Validates the template.

The CLI has been reorganized in to a folder cli splitting up commands into separate files. The continue-update-rollback command has been removed due to improvements in CloudFormation and maintain cli consistency.

---

• Wrote good commit messages.
• Squashed related commits together after the changes have been approved.
• Commit message starts with [Resolve #issue-number] (if a related issue exists).
• Added unit tests.
• Added integration tests (if applicable).
• All unit tests (make test) are passing.
• Used the same coding conventions as the rest of the project.
• The new code doesn't generate flake8 (make lint) offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.

[b-t-g]

Should the reason for this be documented? I'm uncertain.

[theseanything]

Leaving as cli group behaviours are documented with Click documentation

[b-t-g]

Is there another place where this function could be placed?

[b-t-g]

It seems to me like there is a better place for this function than here.

[theseanything]

Can remove all together

[b-t-g]

Is there a good reason for defining this function here and describe.py?

[theseanything]

Yup duplicated function - removed from describe.py and imported from here

[b-t-g]

Name of this file is slightly misleading, since it contains all of the "read only" operations, instead of just the describe commands.

[theseanything]

Yeah - so realised I can't split out "list" commands in to a list.py and import list, because it clashes with built-in python keyword "list". Not sure what to rename too?

[theseanything]

Creating extra list.py file

[b-t-g]

Expanding on this, we discussed offline that we can just do

from list import list_group

for the purposes of using that module within Sceptre. It is more verbose, but it makes the structure of the commands more intuitive and, thus, more maintainable.

This may cause confusion if someone wanted to use Sceptre as a Python module and import the Sceptre list.py; however, we don't officially support using cli commands as modules and users may sidestep this issue by adding the import statement:

import list as list_commands

instead.

[theseanything]

Requires #259 before merging.