add lifecycle.ignore_changes for initial_group_config

[ysuzuki19]

resolves #67

google_cloud_identity_group has optional argument initial_group_config (ref), whose default is EMPTY.

If we dont set this attribute in the terraform-google-modules/group/google module, the default EMPTY value is used internally.
As the result, an imported group (with initlal_group_config=null) will be replace when running terraform apply.

Therefore, I added lifecycle.ignore_changes for this attribute.

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