[mgmt docker] Clean up docker-sonic-mgmt dockerfile

[daall]

• Alphabetize dependencies to prevent duplicates
• Remove uneccesary git clone

Signed-off-by: Danny Allen daall@microsoft.com

- Why I did it
I did it to reduce the size of the sonic-mgmt docker image as well as try to improve build performance.

- How I did it
I did 3 main things:

• I alphabetized the list of apt and pip dependencies to remove duplicates and hopefully prevent duplicates from being added in the future.
• Before we were installing cffi from apt, then purging some of its dependencies, then re-installing it from pip. So, I simplified this process by skipping the step to install cffi from apt and just install it in pip.
• I removed the git clone of sonic-mgmt at the end. Since people mount their own sonic-mgmt for testing, this step is unnecessary and just adds a lot to the size of the docker image.

- How to verify it
Built and ran locally, no issues so far.

- Description for the changelog

Cleans up docker-sonic-mgmt dockerfile

- A picture of a cute animal (not mandatory but encouraged)

[yxieca]

Just read your PR comments again. Not a big deal, but if I do it, I would create 3 PRs.

Or you can stick with one PR but 3 commits to make review easier.

You don't need to split now.

[daall]

retest mellanox please

[daall]

retest vsimage please