Add Cumulus VX 4.3.0, update usage, increase RAM

[antonalekseev]

This PR

• Adds the latest Cumulus VX image version 4.3.0
• Mentions different default login credentials depending on the version, see What is the Default User Name and Password in Cumulus Linux? – Cumulus Networks® Knowledge Base

• Increases RAM for versions >= 4 are not usable with 512M RAM, per this note RE system requirements 768M is the minimum.

  When updating an existing appliance:

• [x] The new version is on top.
• [x] The filenames in the "images" section are unique, to avoid appliances / version overwriting each other.
• [x] If you forked the repo, running check.py doesn't drop any errors for the updated file.

[grossmj]

Thanks!