set MOLECULE_PLATFORM_NAME environment variable when --platform_name is set

[martybarlow-itstack]

One can configure the platform to test by using either environment variable MOLECULE_PLATFORM_NAME or setting the platform_name name switch.

However, to be consistent it would be good if MOLECULE_PLATFORM_NAME environment variable was set if the switch was used.

This would be consistent with the --scenario-name switch and the MOLECULE_SCENARIO_NAME environment variable.

thanks for considering the idea.