Rename methods that return config/vApp/VM schemas

These methods are already namespaced to the class that's defining them. The
prefixes of `provision_` and `customize_` seem rather arbitrary and don't
really describe the contents.

These are about to be refactored to different locations where I'll just call
them `VAPP_SCHEMA` and `VM_SCHEMA`. Calling out this rename in preperation
for that.
Move schemas to constants under Launcher::Schema

This follows the pattern in use by alphagov/vcloud-edge_gateway, which I
like because it makes it clear that these don't get computed or change at
runtime and allows us to point users at them if they need to know what a
valid configuration looks like.

Like alphagov/vcloud-edge_gateway@b30f91b I have also included a link in the
README so that people can find them.

PS: Additional newline in VmOrchestrator to match other classes.
Merge pull request #37 from alphagov/71591898-consistent_schema_locations

[#71591898] Move schemas to constants under Launcher::Schema