Add "dry run" support to apps and actors init workflow

[mwvaughn]

Problem

1. The apps init and actors init workflows send a dynamically populated dictionary of key-value pairs to an instance of cookiecutter() to create a new project from a template. The dictionary is populated by reading the local environment but also by reading the CLI's settings, so from a user's point of view it might not be 100% clear how and what variables are being sent to cookiecutter for interpretation.
2. It is possible to attempt to create a cookiecutter based project if one already exists or in a directory where the user does not have write privileges, leading to errors.

Proposed Solution Implement a "dry run" function, accessible via -R, --dry-run (same as for the deploy workflow) that will a. Present all dynamically generated key:value pairs b. Inspect the filesystem to determine whether it is possible to create a new project

Notes Ideally, identical implementation and behavior will exist between the apps init and actors init implementations. Later on, we may want to factor some of the logic into a parent class.

[mwvaughn]

I will begin work on this in the actors init command

[mwvaughn]

Functionality for apps init is currently as follows on branch feat-350-init-dry-run:

$ tapis apps init APPY --debug --app-version 1.0.0 \
    --app-description "If you are appy and you know it clap your hands" --dry-run
+-------+-----------------------------------------------------------------------------------+
| stage | message                                                                           |
+-------+-----------------------------------------------------------------------------------+
| setup | Project path: ./appy                                                               |
| setup | CookieCutter variable name=APPY                                                    |
| setup | CookieCutter variable description=If you are appy and you know it clap your hands |
| setup | CookieCutter variable version=1.0.0                                               |
| setup | CookieCutter variable project_slug=appy                                            |
| setup | CookieCutter variable docker_namespace=sd2e                                       |
| setup | CookieCutter variable docker_registry=https://index.docker.io                     |
| setup | Workflow stopped because this is a dry run                                        |
+-------+-----------------------------------------------------------------------------------+

[mwvaughn]

Completed and merged as of 18e0b5c9fbc1efb469e229ee074d5df4cb752d3c