Add configuration flags to ddev pull, fixes #405

[andrewfrench]

The Problem/Issue/Bug:

#405

The entire pull process was required for each invocation of ddev pull, which is not ideal when file and database assets can be in the many gigabytes.

How this PR Solves The Problem:

This adds several configuration flags to ddev pull:

• --skip-db does not download or import the remote database backup asset
• --skip-files does not download or import the remote file backup asset
• --env determines the remote environment to pull from
• --skip-confirmation will bypass the interactive confirmation and begin the pull
• --skip-import will download file or database backup assets, but skip the import

Additionally, an update was made to respect the DRUD_NONINTERACTIVE environment variable to bypass confirmation, and is functionally equivalent to the --skip-confirmation flag.

Manual Testing Instructions:

With a remote environment configured, execute ddev pull with combinations of the above flags. These changes are not adding much new functionality other than the ability to configure the environment being pulled from, but are adding options to bypass default steps of the pull process.

• Ensure that the --skip-db flag only pulls the files archive
• Ensure that the --skip-files flag only pull the database archive
• Ensure that the --env flag switched the default environment to be pulled from
• Ensure that the --skip-confirmation flag bypasses the interactive confirmation step
• Ensure that the --skip-import flag only downloads the relevant archives and skips the import step

Automated Testing Overview:

There remain to be no automated tests for ddev pull. I plan to refactor the Provider code into an interface to allow for easy mocking and improved testability.

Related Issue Link(s):

#405

[rfay]

I had no trouble with this or with the code. I tested on macOS Mojave against Pantheon. I tried ddev pull, ddev pull -y, ddev pull --skip-db, ddev pull --skip-files, all behaved as expected. Brad's mom will be very happy.