TME-2307: add support for existing trail with x-account resources

[sarjumulmi]

Add support for existing cloudtrail with resources deployed in different aws accounts.

Changes:

• configuration_alias added for log bucket account. need to pass in the provider alias for log bucket when invoking the module
• conditionally create resources based on is_existing_cloudtrail_cross_account var
• local runtime check for assuring other dependent optional variables are provided when is_existing_cloudtrail_cross_account is set to true.
• new variable for expel assume role name instead of hard coded string. add type defn for tags
• change circle ci config as a workaround for terraform validate bug