zpm: rename a variable in `deploy_project`

Instead of `version`, the variable is renamed to `ui_auth_version`. The
difference here is subtle, because the auth version is used in two ways
in zpm:

1) To authenticate to Swift/ZeroCloud services
2) To populate auth information in UI templates of zapps (at deploy
   time)

The first is determined within the `_get_zerocloud_conn`. The second can
also be determined within `_get_zerocloud_conn`, but can be overridden
by the `--no-ui-auth` command line arg.
zpm: add `_guess_auth_version` function
zpm: integrate `_guess_auth_version`

Currently, only deploy and execute require authentication.
commands: refactor the default auth version stuff

Instead of specifying the default in the command line arg definition,
this is handled else where (see _get_zerocloud_conn and
_guess_auth_version).
test_zpm: improve coverage for _get_zerocloud_conn
Add `--force` flag to `zpm deploy`

This forces/allows deployment to non-empty containers.
test_zpm: rename a variable (trivial change)
zpm: clean up comments in _generate_uploads

This reformats the docstring slightly and removes a misleading comment.
zpm: a more idiomatic way of stripping a file ext
Merge pull request #179 from larsbutler/minor-generate-uploads-fixes

Minor fixes to `_generate_uploads`
Merge pull request #178 from larsbutler/deploy-force

Allow user to force deployment to a non-empty container
Merge pull request #176 from larsbutler/deduce-auth-version

Deduce auth version (from cmd line args / env vars)