Support `early_auth_check` flag in provider config. Closes #777
This change adds a new early_auth_check
flag to the provider configuration.
It defaults to true
which causes the provider to perform a dummy request to the GitLab API to verify the
configuration. If that is not desired it can be set to false
which avoids this call.
This can be in useful if the GitLab instance is created within the same terraform module as this provider
is being used.