update CLI auth interface

[jreiberkyle]

Update the CLI auth interface to the following:

init -> login-key

Usage: planet auth login-key

  Obtain and store api key authorization credentials

Options:
  --help          Show this message and exit.

Usage Examples

value -> print-key

Usage: planet auth print-key [OPTIONS]

  Print the stored api key

  Returns nothing if no api key is stored.

Options:
  --help  Show this message and exit.
Usage Examples
User Story: As a CLI user I would like to get the api key for passing into a docker container.

Base usage:

> planet auth print-key
<api key>

Setting a variable:

> export PL_API_KEY=$(planet auth print-key)

[cholmes]

Note that the docs now need to be updated when this merges.