Use parameter to set CIDR for SSH ingress

[katebee]

Hi team 👋 reading through the template and wanted to offer this PR for consideration.

I would like to be able to pass in the CIDR of my office / VPN to determine where the security group will allow SSH traffic from. 😊

AWS documentation recommends limiting access to common administrative ports to only a specific IP address or range of addresses.

AWS Trusted Advisor will also create a high-risk finding under Security if these ports are open in the Security Groups:

Unrestricted access increases opportunities for malicious activity (hacking, denial-of-service attacks, loss of data). The ports with highest risk are flagged red, and those with less risk are flagged yellow. Ports flagged green are typically used by applications that require unrestricted access, such as HTTP and SMTP.

[manuasir]

This LGTM. Thank you @katebee for this awesome contribution!