Fix attach script loop for satellite host attach

[bhpratt]

Custom code is overwriting part of the attach script in some locations, causing the attach script to fail.

This update provides a change to insert the custom code rather than overwriting a line in the attach script. In addition, it changes else/if loop to case for easier code readability and maintenance.

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Output from acceptance testing:

 make testacc TEST=./ibm/service/satellite TESTARGS='-run=TestAccIBMSatelliteAttachHostScriptDataSourceBasic'
=== RUN   TestAccIBMSatelliteAttachHostScriptDataSourceBasic
--- PASS: TestAccIBMSatelliteAttachHostScriptDataSourceBasic (886.52s)
=== RUN   TestAccIBMSatelliteAttachHostScriptDataSourceBasicCoreos
--- PASS: TestAccIBMSatelliteAttachHostScriptDataSourceBasicCoreos (1172.36s)
PASS
ok  	github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/satellite	2060.180s