Allow for configurable periods in cleanup

[mdfilio]

Currently it's hard coded to only check for 7 days back from current date

https://github.com/rackerlabs/ebs_snapper/blob/master/ebs_snapper/clean.py#L84

It would be nice to allow this be a configurable value instead with a default of 7 days to allow easier cleanup when snapshots don't seem to be removed correctly.

[martinb3]

Perhaps we can make this a command-line option for the CLI.