[BUG] passing None to --vlan on --mod-cloud not working

[grafuls]

• QUADS version (rpm -qa | grep quads and/or quads-cli --version):quads-1.1.5-20210511.noarch
• Python version:Python 3.7.4
• Operating System:Fedora release 29

Describe the bug
When trying to clear a vlan from an existing cloud and as per the documentation, passing None to --vlan argument fails in the parsing of the string into an integer.

To Reproduce
Steps to reproduce the behavior:

1. define a cloud with a vlan_id
2. run mod-cloud against that cloud and pass --vlan None
3. See the stack trace for the exeption

Expected behavior
The vlan should be cleared from the cloud and no error msg should be displayed.

[grafuls]

https://review.gerrithub.io/c/redhat-performance/quads/+/522868