In our discussion with Jussi we come to the conclusion that we want
to verify that all Key attributes contain values in the expected types,
but at the same time, we don't want to focus on validating the semantics
behind them.
The reason is that having a Key instance with invalid attributes is
possible and supported by the spec.
That's why we have a "threshold" for the roles meaning we can have up to
a certain number of invalid Keys until we satisfy
the required threshold.

Also, for deeper semantic validation it's better to be done in
securesystemslib which does the actual work with keys.

For context see: theupdateframework#1438

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

Clarify that we don't semantically validate "Key" instances during
initialization and that this is a responsibility of securesystemslib.

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>