zpm: abstract `_get_zerocloud_client`

This utility can be used elsewhere to handle the various auth args and
create a ZeroCloud client object.
tests/test_zpm: tests for _get_zerocloud_client
miniswift: add `download` method

This method can be used to download single files from a container,
similar to `swift download`.
Implement `zpm execute` command
tests: update cram tests to include `execute` cmd
Merge pull request #115 from larsbutler/zpm-execute

Add zpm execute command