Describe the bug
'brave deploy UNIT --port 8888:8888 --name UNIT' will fail if the port 8888 on the host is already taken up by another process. Result

To Reproduce

1. Run Jupyter Notebook: jupyter notebook
2. In another window, build mlbase-1.0 image brave base github.com/beringresearch/bravefiles/ubuntu/ubuntu-bionic-mlbase
3. Deploy mlbase-1.0 image: `brave deploy mlbase-1.0 --name mlbase --port 8888:8888'

The output is:

Importing mlbase-1.0.tar.gz
Unit launched:  mlbase
Service started:  mlbase
2020/09/17 10:36:30 <nil>

`

Expected behavior
An error should be raised before image begins to be imported to alert the user that a port is in use.

Environment (please complete the following information):

• Your operating system name and version: Ubuntu 18.04
• Bravetools version: 1.53
• LXD version: 4.4