Update dirac-install for DIRACOS2

[chrisburr]

As part of #4732 the dirac-install script has crude support for installing a Python 3 client with DIRACOS2. This should be updated to:

• Use Fix setuptools installation #4774
• Use DIRACOS2 from DIRACGrid
• Support the version of DIRACOS being specified as well as extensions

[chrisburr]

I think this might not be needed and instead we can just drop dirac-install.py as part of the Python 3 migration.

The install steps with DIRACOS2 are:

$ wget https://github.com/DIRACGrid/DIRACOS2/releases/download/2.0a8/DIRACOS-Linux-x86_64.sh
$ bash DIRACOS-Linux-x86_64.sh

(once we make the first release we can use "latest" instead of a specific version)

The installer then tells you what to do:

DIRACOS has been installed sucessfully in xxxxxx/diracos

 * It can now be activated with:
       source xxxxxx/diracos/diracosrc

 * To install vanilla DIRAC then run:
       pip install DIRAC

   Alternatively, to install a specific version:
       pip install DIRAC==7.2.0a34

   Alternatively, to install a DIRAC extension, install the associated Python package. E.g. for LHCbDIRAC run:
       pip install LHCbDIRAC

 * You can then get the configuration for your DIRAC installation using (chnaging MY_SETUP and MY_CONFIGURATION_URL as appropriate):
       dirac-proxy-init --nocs
       dirac-configure -S MY_SETUP -C MY_CONFIGURATION_URL --SkipCAChecks
       dirac-proxy-init

For more advanced installation instructions see: TODO

The only change I'd still like to make is to change configuration step to be more intuitive. My suggestion would be to make it so that dirac-configure without any arguments runs a small wizard that asks for a setup and configuration URL using metadata from the extension_metadata entrypoint. e.g. if LHCbDIRAC is installed the completed wizard would look something like:

$ wget https://github.com/DIRACGrid/DIRACOS2/releases/download/latest/DIRACOS-Linux-x86_64.sh
$ bash DIRACOS-Linux-x86_64.sh
$ source diracos/diracosrc
$ dirac-configure
Select a setup:
 - [x] LHCb-Production (default)
 - [ ] LHCb-Certification
 - [ ] Other
Configuration URL [default: dips://lhcb-conf-dirac.cern.ch:9135/Configuration/Server]: 

Generating proxy... (you will be prompted for your password)
Enter Certificate password:
Configuration downloaded successfully 

[marianne013]

I don't like wizards, they are hard to script.

[chrisburr]

The original options will still be there, just if you pass nothing it will guide you through.

[chrisburr]

Closing as dirac-install.py won't be required, as described in #4775 (comment).