Allows skipping fabricmanager download and installation

[engel75]

What this PR does / why we need it:

For some driver versions no Fabric Manager does exist, eg. 550.90.07 and for smaller GPUs the fabric manager is not needed at all. So we need the possibility to skip the fabricmanager download and installation.

This is what this PR does. It skips the download if the file does not exist, and it also skips the installation if the archive does not exist.

Which issue(s) this PR fixes:
Fixes #26

Special notes for your reviewer:

Release note:

allow skipping download and installation of the fabricmanager if no compatibe version exists.

[dhague]

Thanks for the PR - I agree that the fabricmanager download should be optional, but I would prefer that this is made explicit rather than merely skipping if it doesn't exist for that version. We often do need the fabric manager, and I would want the build to fail if there is none for that driver version.

Could you please amend the PR so that it uses a build argument instead, e.g. --build-arg FABRIC_MANAGER=false, to explicitly skip the download & installation of the fabric manager.