fixes required to avoid breaking Cray support

[boegel]

Two issues popped up when testing current develop on CSCS' Piz Daint:

• The CrayToolchain easyblock needs a tweak to avoid loading the modules for the dependencies specified in easyconfig file to install a new toolchain (for example CrayGNU or CrayIntel)

  This is required because of the change in semantics of the dummy toolchain (which is now an alias for the new system toolchain), where using a dummy version no longer implies not loading the modules for the toolchain and dependencies in the build environment (see also deprecate 'dummy' toolchain, replace with 'system' toolchain #2877).

  Doing so is a) not needed at all to install a Cray* toolchain, b) requires more than a simple module load, since it's likely that modules that loaded in the default environment on a Cray system will trigger a conflict, for example:

  Module 'PrgEnv-gnu/6.0.4' conflicts with the currently loaded module(s) 'PrgEnv-cray/6.0.4'")
  

  The prepare_step has been updated in 42a166b to allow not loading the modules for the toolchain & dependencies (the default is of course unchanged), which can be leveraged easily in the CrayToolchain easyblock.

• The output of modulecmd python load sometimes includes a faulty source command using some versions of Cray's environment modules 3.2.10.x (certainly with modules/3.2.10.6).

  The run_module method of EnvironmentModulesC class has been customized to strip out these faulty source commands, which fixes problems like:

  Changing environment as dictated by module failed: invalid syntax (<string>, line 1) (stdout: source /opt/cray/pe/modules/3.2.10.6/init/bash
  source /opt/cray/pe/modules/3.2.10.6/init/bash
  

cc @gppezzi @victorusu

[bartoldeman]

lgtm