Add CI job to run unittests under MPI

[mtreinish]

Summary

In #1242 we added a basic smoke test job for MPI builds using standalone mode. That job builds aer in standalone mode with MPI and then runs a qobj of a grovers circuit with mpi enabled and validate it works (and the result metadata has the mpi fields set). That was useful to provide a quick smoke test to verify that a proposed change doesn't completely break MPI builds. However, it doesn't fully validate aer's usage with MPI enabled. This commit adds a new CI job that runs the full python unittest suite under mpi run using a aer binary built with mpi support. The tradeoff there though is we have to run the tests serially using the stdlib python unittest runner (since stestr runs tests in parallel processes) the process is running under mpirun.

Additionally, this commit bumps the timeout for the unit test jobs to 1 hour as we've been regularly hitting the timeout on the CI test jobs. The timeout for the new mpi job is double that (120 min) since it is running tests serially instead of in 2 or 4 processes.

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