Add stack prefix wildcard support for snapshot purging

[cliffano]

Currently Stack Manager snapshots purging supports StackPrefix parameter, but this is not good enough because a team wants to manage a number of environments from a single stack manager. Other than that, another problem is that stack prefix is often not known during the creation of Stack Manager.

We need to introduce a stack prefix wildcard support. This way user can:

1. define a stack prefix wildcard, e.g. myorg-* , which will then only purge snapshots taken from environments with stack prefix which fits the wildcard regex
2. leave the wildcard as empty, which will then purge the snapshots across the account

This new stack prefix wildcard support will provide a boundary support for Stack Manager, essentially allowing multiple teams to have one Stack Manager each, with the wildcard as the boundary.