gnpy currently uses the same parameter for tx output power and span
input power: this prevents from modelling low tx power effect.
This patch introduces a new tx-cannel-power and uses it to
propagate in ROADM.
Signed-off-by: EstherLerouzic <esther.lerouzic@orange.com>
Change-Id: Id3ac75e2cb617b513bdb38b51a52e05d15af46f5