Label support for controlled unitary

[1ucian0]

@ajavadia found the following issue in master:

from qiskit import QuantumCircuit
from qiskit.extensions import UnitaryGate

circ = QuantumCircuit(2)
u = UnitaryGate([[1,0], [0,1]], label='my gate')
circ.compose(u.control(1), [0,1], inplace=True)
print(circ)

q_0: ───────■────────
     ┌──────┴───────┐
q_1: ┤ circuit57_dg ├
     └──────────────┘

With this PR, here is the output:

q_0: ─────■─────
     ┌────┴────┐
q_1: ┤ my gate ├
     └─────────┘