Misc improvements/fixes

Most visible change:

• TargetConf have been renamed to TestEnvConf to match its actual usage. Top-level key in YAML file has changed accordingly to 'testenv-conf'
• Target connection settings are now located under a "target" subkey in TestEnvConf:

testenv-conf:
    target:
        name: foo
        host: foobar
        password: mypassword
        username: foobar

The same changes applies to the dictionary passed to TestEnvConf.__init__