support deployment with forked git repo url

[allinwonder]

The current deployment is good that it creates a set of default rules, but to allow customers and workshop attendees to extend these rules for their needs, we can allow them to use their repo as the source repo.

This pull request is allowing attendees to create the stack with a git repo parameter. For example using command rake jenkins:create['0.0.0.0/0','[email protected]:allinwonder/aws-devsecops-workshop.git'] will create a stack looking into the source repo from "https://github.com/allinwonder/aws-devsecops-workshop/"

[nand0p]

GITHUB_OWNER and GITHUB_BRANCHvars are now set at the top of deploy.sh for simplicity of one-button command deployment. latest amazon linux ami contains ssm agent by default.