job: update X-Fleet syntax

summary:
	X-Conflicts => Conflicts
	X-ConditionMachineOf => MachineOf
	X-ConditionMachineID => MachineID
	X-ConditionMachineMetadata => MachineMetadata

The metadata (as returned by job.RequiredTargetMetadata) changes from a
map[string][]string to map[string]pkg.Set, since really the order of the
values for a particular key are unimportant. This necessitates changes
in machine.HasMetadata.

Test coverage is cleaned up and improved across the board.
docs: update documentation to reflect prefix removal
Merge pull request #621 from jonboulle/621_remove_x_prefix

deprecate `X-` for options in `X-Fleet` section