Fix misleading warnings

[tylergmuir]

This fixes 2 different warning that you receiving when calling the modules in this collection.

First is the warning that you receive when using user/password authentication instead of token-based authentication. In the case they you do not use user/password authentication by not specifying the parameters user and password you still receive a warning due to the check looking for only in the case that both user and password are empty strings. This PR fixes this by replacing those checks against empty strings for a check against None.

The second warning occurs if you use the parameter common_name. Since the parameter common_name has an alias that matches the name, you receive a warning from Ansible saying that you are using common_name and its alias common_name. This removes the duplicate alias to remove that warning.

[rvelaVenafi]

Hello there @tylergmuir I know this PR is quite old, but I wanted to revisit it.
First, thank you very much for your contribution, we at Venafi really appreciate the community comming together to improve the tools we offer.

Secondly, For the first warning you described (user/auth warning), could you provide a sample playbook that reproduces the issue?

Third, we need your commits to be signed in order to merge to our main branch. So three options here:
a) you can sign the commits of this PR, which is a bit of an effort (not straighforward), or
b) you can open a new PR with the same code but with the commits signed this time, or
c) I can do b) myself.

Let me know how you want to proceed.

Thanks in advance!