Synthetics - Make Secure Credential creation optional

[bpeckNR]

Summary

Currently, migratemonitors.py creates empty shells of secure credentials in the target account when migrating Synthetic monitors.

Desired Behavior

The creation of secure credential shells should be optional and controlled by an argument passed into the migratemontiors.py script. The credentials might be created manually before migrating monitors between accounts, especially since we cannot programmatically migrate credential values.

Possible Solution

Add an argument to skip secure credential creation.

Additional context

Manual work is required to populate secure credentials with their values. Some customers may want to recreate the credentials manually instead of creating empty shells during an automated migration.