Fix: Sequence Template Issue deployed in gateway

[BLasan]

Description

When a sequence with <sequence name="sequence"> is uploaded, it gives an error on the API invocation. This is due to a missing sequence file with the given name. Previously, this content was embedded inside a <sequence> tag with the help of a j2 template. With the new implementation, there's no need of a template. First the logic checks whether the given file has a <sequence> tag as its root element. If presents, then it will update its name attribute. If tag is not present, it will add the hardcoded <sequence> tag and then build the OMElement using it. This way, it prevents having two sequence tags unnecessarily which causes issues during the sequence engagement.

Issue

• Provide better documentation with sample for Sequence BE api-manager#3448

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