OBS: fix package for Fedora

Fedora distribution refuses shebang with unknown python version:
ERROR: ambiguous python shebang in /opt/slapos/tmp-networkcached/bin/networkcache-download: #!/usr/bin/python. Change it to python3 (or python2) explicitly.
ERROR: ambiguous python shebang in /opt/slapos/tmp-networkcached/bin/networkcache-upload: #!/usr/bin/python. Change it to python3 (or python2) explicitly.
ERROR: ambiguous python shebang in /opt/slapos/tmp-networkcached/bin/buildout: #!/usr/bin/python. Change it to python3 (or python2) explicitly.
ERROR: ambiguous python shebang in /opt/slapos/tmp-networkcached/bin/generate-signature-key: #!/usr/bin/python. Change it to python3 (or python2) explicitly.

Using python2.7 explicitely will make the shebang /usr/bin/python2.7

obs/slapos/prepare_download_cache.sh

@@ -27,7 +27,7 @@ sed  "s/\%RECIPE_VERSION\%/$RECIPE_VERSION/g;s|\%PATCHES_DIRECTORY\%|$PATCHES_DI
 cd $NETWORKCACHED_DIRECTORY
 # Download  bootstrap file
 wget https://bootstrap.pypa.io/bootstrap-buildout.py -O bootstrap.py
-(python -S bootstrap.py --buildout-version $BUILDOUT_VERSION \
+(python2.7 -S bootstrap.py --buildout-version $BUILDOUT_VERSION \
                            --setuptools-to-dir eggs \
                            -f http://www.nexedi.org/static/packages/source/slapos.buildout/ && \
     ./bin/buildout -v)