Improve series upgrade handling by allowing for manual venv rebuild

It is likely that an admin will want to validate that the charm code will be successful after a series upgrade before issuing the `juju upgrade-series complete` command. However, the [current logic](https://github.com/juju-solutions/layer-basic/blob/master/lib/charms/layer/basic.py#L116-L119) will [only trigger on the `post-series-upgrade` hook](https://github.com/juju-solutions/layer-basic/blob/master/lib/charms/layer/basic.py#L102) which doesn't allow the admin a chance to validate the new venv.

I think we should add a `--rebuild` option to the [`charm-env` command](https://github.com/juju-solutions/layer-basic/blob/master/bin/charm-env) which does the venv rebuild manually and presumably sets a flag to indicate to the bootstrap code that it has already been rebuilt for the current series so that it doesn't get rebuilt again upon `post-series-upgrade`.