While the use of sudo (made possible by 53cf084) allows to install
additional packages, the warning copied below was still reported.
To avoid this warning and streamline the installation of new packages,
this commit (1) introduces the concept of "pre_exec" entrypoint hook
and (2) adds such a hook to change the ownership of python "bin" and
"site-packages" directories for the manylinux images.
Warning reported are similar to this one:
```
The directory '/home/jcfr/.cache/pip/http' or its parent directory is
not owned by the current user and the cache has been disabled. Please
check the permissions and owner of that directory. If executing pip
with sudo, you may want sudo's -H flag.
```
Note that the sudo "-H" flag suggested in the warning is not available
in centos.
This will allow build script like "build-wheels.sh" to run command like
the following (see [1]):
sudo yum install -y atlas-devel
It will also allow to pip install using sudo and avoid error reported in [3].
That said, instead of running pip using sudo, the recommended approach
is to specify the "--user" flag [2]
[1] 893d92517e/travis/build-wheels.sh
[2] http://stackoverflow.com/questions/7143077/how-can-i-install-packages-in-my-home-folder-with-pip#7143496
[3] Error reported when running pip without "sudo pip install <packagename" or "pip install --user <packagename>"
```
Exception:
Traceback (most recent call last):
File "/opt/_internal/cpython-2.7.11-ucs2/lib/python2.7/site-packages/pip/basecommand.py", line 215, in main
status = self.run(options, args)
File "/opt/_internal/cpython-2.7.11-ucs2/lib/python2.7/site-packages/pip/commands/install.py", line 317, in run
prefix=options.prefix_path,
File "/opt/_internal/cpython-2.7.11-ucs2/lib/python2.7/site-packages/pip/req/req_set.py", line 736, in install
requirement.uninstall(auto_confirm=True)
File "/opt/_internal/cpython-2.7.11-ucs2/lib/python2.7/site-packages/pip/req/req_install.py", line 742, in uninstall
paths_to_remove.remove(auto_confirm)
File "/opt/_internal/cpython-2.7.11-ucs2/lib/python2.7/site-packages/pip/req/req_uninstall.py", line 115, in remove
renames(path, new_path)
File "/opt/_internal/cpython-2.7.11-ucs2/lib/python2.7/site-packages/pip/utils/__init__.py", line 267, in renames
shutil.move(old, new)
File "/opt/_internal/cpython-2.7.11-ucs2/lib/python2.7/shutil.py", line 303, in move
os.unlink(src)
OSError: [Errno 13] Permission denied: '/opt/_internal/cpython-2.7.11-ucs2/bin/easy_install'
```
