Currently JoinConfigFileAndDefaultsToInternalConfig is doing a couple of
different things depending on its parameters. It:

- loads a versioned JoinConfiguration from an YAML file.
- returns defaulted JoinConfiguration allowing for some overrides.

In order to make code more manageable, the following steps are taken:

- Introduce LoadJoinConfigurationFromFile, which loads a versioned
  JoinConfiguration from an YAML file, defaults it (both dynamically and
  statically), converts it to internal JoinConfiguration and validates it.

- Introduce DefaultedJoinConfiguration, which returns defaulted (both
  dynamically and statically) and verified internal JoinConfiguration.
  The possibility of overwriting defaults via versioned JoinConfiguration is
  retained.

- Re-implement JoinConfigFileAndDefaultsToInternalConfig to use
  LoadJoinConfigurationFromFile and DefaultedJoinConfiguration.

- Replace some calls to JoinConfigFileAndDefaultsToInternalConfig with calls to
  either LoadJoinConfigurationFromFile or DefaultedJoinConfiguration where
  appropriate.

- Rename JoinConfigFileAndDefaultsToInternalConfig to the more appropriate name
  LoadOrDefaultJoinConfiguration.
Signed-off-by: Rostislav M. Georgiev <rostislavg@vmware.com>