Update the CNOT-Dihedral class for RB

[ShellyGarion]

What is the expected behavior?

Update the code for the CNOT-Dihedral class in RB:
https://github.com/Qiskit/qiskit-ignis/blob/master/qiskit/ignis/verification/randomized_benchmarking/dihedral.py

in order not to use pre-generated tables (pkl file) for the 2-qubit CNOT-Dihedral group
(this pkl file make the tests fail sometimes).

Use similar API to the Clifford operator class in Terra (as much as possible):
Qiskit/qiskit#3938