As of Jenkins core has been changing the terminology from "Slave" to "Agent", and plugins should follow this, I create this ticket to rename the "Slave" references on the ec2-plugin to "Agent".

Acceptance Criteria:

• There are no "Slave" references on source code
• There are no "Slave" references on documentation

We could still keep a link in the documentation to this ticket or the EPIC so people looking for the term "Slave" can find that in the future just in case they have no idea about this renaming.