sql backup policy

[bcullman]

Background
In some circumstances the pattern matching was too greedy, resulting in additional non-related backfiles being removed.

With this change, harden the ApplyRetentionPolicy to make it more reliable. additionally update tests to account for files that should not be removed.

Pre-requisites
[NA] Id should be a GUID that is not 00000000-0000-0000-0000-000000000000
NOTE If you are modifying an existing step template, please make sure that you do not modify the Id property (updating the Id will break the Library sync functionality in Octopus).
[NA] Version should be incremented, otherwise the integration with Octopus won't update the step template correctly
[NA] Parameter names should not start with $
[NA] Step template parameter names (the ones declared in the JSON, not the script body) should be prefixed with a namespace so that they are less likely to clash with other user-defined variables in Octopus (see OctopusDeploy/Issues#2126). For example, use an abbreviated name of the step template or the category of the step template).
[NA] LastModifiedBy field must be present, and (optionally) updated with the correct author
[✔] The best practices documented here have been applied
[NA] If a new Category has been created:
[NA] An image with the name {categoryname}.png must be present under the step-templates/logos folder
[NA] The switch in the humanize function in gulpfile.babel.js must have a case statement corresponding to it