[dds instance]change type of KeepDays to *int and add param Period in BackupStrategy

[Jason-Zhang9309]

What this PR does / why we need it:
change type of KeepDays to *int, so the value can be set to 0
add param Period, so the BackupStrategy struct can be used while updating backup_strategy through the backups/policy API

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

Release note:

[coveralls]

Pull Request Test Coverage Report for Build 1228

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 68.32%

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Totals
Change from base Build 1227:	0.0%
Covered Lines:	11654
Relevant Lines:	17058

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💛 - Coveralls