deployment: make cap-client available on lxplus by a source script

[lukasheinrich]

Most physics users will expect that the cap client is somehow usable on LXPLUS so if the cap-client could somehow be deployed there (via RPM or some virtualenv that can be sources) that would be nice

[tiborsimko]

I have installed it on the LXPLUS7 so that users could do:

$ ssh lxplus7.cern.ch
$ source ~simko/public/cap/bin/activate
$ cap-client --help

Similarly to how people can use reana-client.

Note that people have to use LXPLUS7 and not LXPLUS yet because of:

• LXPLUS's default version is Python-2.6 which is not compatible with CAP;
• trying Python-2.7 via software collections on LXPLUS leads to python library location troubles;
• trying Python-3 via software collections on LXPLUS (that works nicely for REANA) wouldn't work for CAP client yet because of minor Python 3 support issues (e.g. see Python 3 compatibility #81 about urlparse which is still there)

So while the above way of cap-client on LXPLUS7 is fully OK, we can improve things still...