r/security_list: Suppress case diffs for security_list

[grubernaut]

With outbound_cidr_policy and policy attributes, the API returns a fully upper-case value regardless of a lower-case value being supplied. Needed to suppress the diff with the updated value to ignore case.

Fixes: #17

[grubernaut]

$ make testacc TEST=./opc TESTARGS="-run=TestAccOPCSecurityList"
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./opc -v -run=TestAccOPCSecurityList -timeout 120m
=== RUN   TestAccOPCSecurityList_importBasic
--- PASS: TestAccOPCSecurityList_importBasic (12.51s)
=== RUN   TestAccOPCSecurityList_importComplete
--- PASS: TestAccOPCSecurityList_importComplete (12.70s)
=== RUN   TestAccOPCSecurityList_basic
--- PASS: TestAccOPCSecurityList_basic (12.01s)
=== RUN   TestAccOPCSecurityList_complete
--- PASS: TestAccOPCSecurityList_complete (16.06s)
=== RUN   TestAccOPCSecurityList_lowercasePolicies
--- PASS: TestAccOPCSecurityList_lowercasePolicies (13.05s)
PASS
ok  	github.com/terraform-providers/terraform-provider-opc/opc	66.335s

[mbfrahry]

LGTM!