add --retries/-r root flag and configure the SSM client to retry

[toshok]

we at honeycomb are running up against aws throttling DescribeParameter api calls (issue #34)

This diff adds a flag for max-retries (--retries/-r). The default value is 10, which seems to fix things for 15 simultaneous runs of while :; do chamber exec dev -- echo hi ; echo bye; done.

The flag is attached to the RootCmd, so invoking it will look something like:

$ chamber --retries 100 exec something -- command
$ chamber -r 100 exec something -- command

numRetries is passed to all store.NewSSMStore calls, which doesn't feel very golang-like. Maybe I should add a store.StoreConfig struct? Let me know!

[dfuentes]

Looks great, thanks for the improvement!