Expand PATH to find Python/Pip executables

[sbesson]

See Homebrew/homebrew-core#14408

Following a recent change in Homebrew core, the python executable is no longer available under the PATH by default. This PR implements the workaround used in Homebrew/brew#2897 to minimize the number of changes in the code.

An alternative might be to use a virtual environment using the system Python i.e.

sudo easy_install pip
sudo pip install virtualenv
virtualenv /tmp/omero_venv
source /tmp/omero_venv

To be reviewed with @jburel in the context of OMERO 5.4.0

[jburel]

With the work on the documentation see ome/omero-documentation#1766
I will be in favour of doing in the minimum here and revisit the actual commands in this repo or deprecate that section
We have never used the code snippets in the doc.

[joshmoore]

@jburel: do you mean you'd take this PR as is or close?

[jburel]

I will take as it is. We no longer refer to it in our doc following the work by @will-moore and @rgozim
and I will be in favour of deprecating that section

[joshmoore]

Works for me. Can discuss the future of the /osx/ scripts elsewhere.