Bug 1828981

Description of problem:

Lot of examples use the same engine configuration options:
- engine url (e.g. https://my-engine/
- username (e.g. admin@internal)
- password 
- cafile (e.g. /etc/pki/vdsm/certs/cacert.pem on a host)
- insecure (do not verify server certs)

Most examples require changing the example code to set the right parameters.
This means that the examples files are likely not tested properly because
testing require modifying the code before you post a patch. Since there are
no tests for the example code, it is likely to be broken.

Some examples (upload_disk.py, download_disk.py, backup_vm.py) provide
a command line interface to set these values:

> python3 /root/ovirt-engine-sdk/sdk/examples/upload_disk.py \
>     --engine-url "https://`hostname`/" \
>     --username admin@internal \
>     --password-file pssword  \
>     --cafile /tmp/ca.pem \

This make these examples testable and real useful command line tool, and
these are very useful for developers.

However for users such long command line is too much pain to use. We need 
to move the common engine config to single configuration file, so we can
use:

    python3 /root/ovirt-engine-sdk/sdk/examples/upload_disk.py \
        -c my-engine.conf

Or even:

    python3 /root/ovirt-engine-sdk/sdk/examples/upload_disk.py ...

Reading the configuration file from documented location like:
~/.config/ovirt-engine-sdk/my-engine.conf


We need common code which should be part of the SDK for reading the
configuration file and for creating a command line parser populated
with the common options that all examples should have.

    from sdk4 import cli

    def main():
        # Return argparse.ArgumentParser populated with common options.
        parser = cli.argument_parser()

        # Add example specific arguments
        parser.add_argument("--foo", help="specific example argument")

        args = parser.parse_args()

        # Create connections from standard configuration file.
        connection = sdk.Connection.fromconf(args.engine_config)

Configuration file format:

# ovirt-engine-sdk configuration file

[engine]
# engine url without the "ovirt-engine/" path component.
url = https://my.engine/

# supported types:
# password:value - embed password in the config file (insecure)
# file:filenae - read password from file
# leave empty to read password from stdin (using getpass)
password = 

# engine username
username = admin@internal

# cafile - path to engine certificate
# can be downloaded from
# https://my.engine/ovirt-engine/services/pki-resource?resource=ca-certificate&format=X509-PEM-CA
# Leave empty to use host trusted CA
cafile = 

# insecure - do not verify server certificate (not recommendded)
insecure = false


The default search path for the configuration can be:
.config/ovirt-engine-sdk/engine.conf

User can keep multiple configurations if they need to work with multiple
engines.


With this it should be easy to add testable and useful examples without 
duplicating the boilerplate for every example.

This can also help users to write consistent command line tools using
ovirt engine sdk.


This can be implemented by for other language bindings.