Fixed centralize logic TODOs in ec2

[tomahawk360]

This PR makes the following changes:

• Replaced three security_groups.filter() instances at lines 180, 197 and 379, each marked with a "Centralize logic..." TODO comment, with get_security_groups() function.
• Removed "Centralize logic..." TODO comments in all previously mentioned instances.

I tested this PR by:

• Launching a cluster
• Adding slaves
• Removing slaves
• Destroying a cluster
• Running "pytest" command

Fixes #377.

[nchammas]

Thanks for taking this on @tomahawk360 and sorry about the delay in reviewing this. I've built on your work by organizing a few other instances of repeated security group logic into a couple of small functions.

The security group code overall is still messy, but I think it's a bit better with these changes.