support relocated python installs, closes #127

[bd4]

No description provided.

[bd4]

Tested by building python 3.4.9 and 2.7.15 with these options:

./configure --prefix=/opt/python/3.4.9 --enable-shared \
   LDFLAGS=-Wl,-rpath,\'$$ORIGIN/../lib\',--enable-new-dtags

and then moving to /opt/python/3.4.9_relocated (and similar for 2.7.15). Then built stc and turbine with these python versions (referenced via custom etc/spack/packages.yaml under spack base), and a normal system python (3.7.0 on manjaro), and ran simple test cases as described here http://swift-lang.github.io/swift-t/guide.html#leaf_python with swift-t command after loading the correct spack modules.

Here is an example spack packages.yaml for reference:

packages:
    python:
        paths:
            python@2.7.15: /opt/python/2.7.15_relocated
            python@3.4.9: /opt/python/3.4.9_relocated
            python@3.7.0: /usr

[bd4]

@bnikolic if you have some time can you test the modified python-config.py from this PR: #159?

[bnikolic]

Thanks for the effort on this! I'll try it out in the next few days