HTools.Backend.Text: Allow "forthcoming" instances

The text format is extended by a column; "Y" stands for "the instance
is forthcoming" and "N" stands for "the instance is real".

Instances in older versions of the data format are treated as real
since so far instances could not be forthcoming.

Signed-off-by: Niklas Hambuechen <niklash@google.com>
HTools: Node violation checks for forthcoming instances

This implements basic support for forthcoming instances in htools,
in particular whether a forthcoming instance to be placed fits on it
and the maintainance of the relevant statistics to decide this.

Each statistic in the `Node` data type that has to handle forthcoming
instances gets a `somethingForth` equivalent that includes the
corresponding `something` statistic for real instances.

For example, when `fMem` measures the free memory taking all real
instances into account, `fMemForth` is the free memory on the node
in case all forthcoming instances on the node were also made real,
thus being less or equal than `fMem` at any given time.

This commit does NOT yet
- handle the maintenance of rMemForth, failN1Forth, and peersForth,
  so n+1 safety for forthcoming instances is not yet implemented
- add the new `somethingForth` fields to the cluster balancedness
  measure (`detailedCVInfoExt`)
- add `test/data/htools/*.json` or `*.data` files that test this
  feature

All unit tests pass.

Signed-off-by: Niklas Hambuechen <niklash@google.com>