[feat] Support for pdsh and clush parallel launchers

[brandonbiggs]

pdsh and clush are parallel ssh tools. I found the regular ssh launcher to be a bit limiting when trying to run tests on multiple nodes. These launchers add support for additional parallel tools to simplify basic ssh tests on many nodes.

[jenkins-cscs]

Can I test this patch?

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (f49b0ec) 86.96% compared to head (9d903c8) 86.97%.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2894   +/-   ##
========================================
  Coverage    86.96%   86.97%           
========================================
  Files           60       60           
  Lines        11588    11596    +8     
========================================
+ Hits         10078    10086    +8     
  Misses        1510     1510           

Impacted Files	Coverage Δ
reframe/core/backends.py	93.93% <ø> (ø)
reframe/core/launchers/rsh.py	100.00% <100.00%> (ø)

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[vkarak]

We should also expand the unit tests in test_launchers.py by extending the fixture here and adapt the remaining unit tests.

I could have a look into that.

[vkarak]

I've added unit tests and moved the definitions of the new launchers in the same file with the ssh launcher, which I renamed to rsh.py for "remote shell launchers". With the minor doc fix above, I think it's ready to go.

[vkarak]

Lgtm!