Not working with conda-build!=1.18.2

[bjlittle]

A tale of woe ...

Require to update centrally-managed-conda to work with more recent versions:

For example, using conda-build-2.0.6-py27_0, you have to set CONDA_NPY, but then the following exception is raised:

TASK: Update source cache
Traceback (most recent call last):
  File "/var/tmp/itwl/conda_cray/tmp_envs/a8662ca63208c6513172/lib/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/var/tmp/itwl/conda_cray/tmp_envs/a8662ca63208c6513172/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/var/tmp/itwl/conda_cray/tmp_envs/a8662ca63208c6513172/lib/python2.7/site-packages/centrally_managed_conda/fetch_sources.py", line 8, in <module>
    from conda_build import external
ImportError: cannot import name external
CalledProcessError: Command '['bash', '/home/h05/itwl/projects/git/configuration/cray/conda_build/update_channel.sh']' returned non-zero exit status 1

The conda-build API has moved on ... i.e. in fetch_sources.py replace from conda_build import external with import conda_build.os_utils.external as external and the function execute no longer exists in conda_build.utils ... there may be more issues to unravel here.