Fix vyos.vyos.vyos_firewall_rules state: replaced to match documentation (#203)

Fix vyos.vyos.vyos_firewall_rules state: replaced to match documentation

SUMMARY
vyos.vyos.vyos_firewall_rules should only try to change listed firewall rules, as documented, when the state is set to replaced. As currently implemented (prior to this PR), it better matches what overridden is meant to do.
Fixes #201
ISSUE TYPE

Bugfix Pull Request

COMPONENT NAME
vyos.vyos.vyos_firewall_rules
ADDITIONAL INFORMATION

Cleanup and document existing code for clarity
Add a failing idempotent test
Add a failing change test
Fix failing tests
Add change fragment

Reviewed-by: GomathiselviS <None>
Reviewed-by: Shawn Wilsher <None>
Reviewed-by: None <None>