Update install script for agent version 2.x

[Shplorf]

Changes:

• Updated agent setup/configure syntax to work for agent 2.x
• Added threatstack_url to allow for internal testing on non-prod environments
• Using different repo for 2.x

Tested on:

• amzn-201709
• amzn-201803
• amzn2-20181024
• centos-7
• debian-8-7
• debian-9-3
• rhel-7-3
• ubuntu-1404
• ubuntu-1604
• ubuntu-1804

[Shplorf]

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