hooks: make the Python interpreter for Git hooks configurable as 'git_hook_interpreter' (Issue #333)

Commit 5e501b6ee639 introduced the use of 'sys.executable' as interpreter
for git hooks instead of 'python2' with the following argument:

"Windows doesn't necessarily have "python2" available in $PATH, but we
still want to make sure we don't end up invoking a python3. Using the
absolute path seems more safe."

But, sys.executable does not necessarily point to Python. When Kallithea is
started under uWSGI, sys.executable points to the uwsgi executable. As a
result, the interpreter encoded in the git hooks on the server repositories
would be:

#!/path/to/uwsgi

And pushing to such repo would result in following client errors:

$ git push
Password for 'http://user@localhost:5050':
Enumerating objects: 3, done.
Counting objects: 100% (3/3), done.
Writing objects: 100% (3/3), 241 bytes | 241.00 KiB/s, done.
Total 3 (delta 0), reused 0 (delta 0)
remote: unable to load configuration from hooks/pre-receive
To http://localhost:5050/gitrepo-new
! [remote rejected] master -> master (pre-receive hook declined)
error: failed to push some refs to 'http://user@localhost:5050/gitrepo-new'

Fix this problem by introducing a configuration setting 'git_hook_interpreter'
that allow administrators to specify which Python interpreter to use.

A subsequent commit will cause its value to be filled in automatically when
generating a new ini file, but an administrator can always override it.