provider/ec2: allocate all instance stores

When starting an instance in EC2, specify all of the
instance stores in the block device mapping. The instance
started will have as many four attached, according to
the instance type.

getBlockDeviceMappings will be enhanced in the near
future to support attaching additional volumes to
instances at creation time (i.e. for charm storage).
Update juju/utils in dependencies.tsv

Update to get ParseSize.
Add a Workspace type.
Use utils/tar.FindFile.
Add state.backups.metadata.NewFromJSONBuffer.
Fix NewArchive.
Add a Metadata method to archive workspaces.
Factor out a basic workspace initializer function.
Remove superfluous blank lines.
Remove return arg names.
Remove a superfluous comment.
Clean up.
Fix a few doc comments.
Archive -> *Archive.
Flatten out and explain a nested if statement.
Inline the NewArchive call.
Drop the rootDir field.
Mention what a blank filename means.
NewWorkspace -> NewWorkspaceFromFilename.
NewWorkspaceFromFile -> NewWorkspaceReader.