Ignore SSM parameter overwrite change

[julienduchesne]

Fixes #3181

Changes proposed in this pull request:

• Allows to go from overwrite "true" to overwrite "false"

Output from acceptance testing:

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./... -v -run=TestAccAWSSSMParameter -timeout 120m
?       github.com/terraform-providers/terraform-provider-aws   [no test files]
=== RUN   TestAccAWSSSMParameter_importBasic
--- PASS: TestAccAWSSSMParameter_importBasic (24.35s)
=== RUN   TestAccAWSSSMParameter_basic
--- PASS: TestAccAWSSSMParameter_basic (21.79s)
=== RUN   TestAccAWSSSMParameter_disappears
--- PASS: TestAccAWSSSMParameter_disappears (13.74s)
=== RUN   TestAccAWSSSMParameter_update
--- PASS: TestAccAWSSSMParameter_update (52.23s)
=== RUN   TestAccAWSSSMParameter_updateDescription
--- PASS: TestAccAWSSSMParameter_updateDescription (41.41s)
=== RUN   TestAccAWSSSMParameter_changeNameForcesNew
--- PASS: TestAccAWSSSMParameter_changeNameForcesNew (37.97s)
=== RUN   TestAccAWSSSMParameter_fullPath
--- PASS: TestAccAWSSSMParameter_fullPath (21.57s)
=== RUN   TestAccAWSSSMParameter_secure
--- PASS: TestAccAWSSSMParameter_secure (20.18s)
=== RUN   TestAccAWSSSMParameter_secure_with_key
--- PASS: TestAccAWSSSMParameter_secure_with_key (55.68s)
=== RUN   TestAccAWSSSMParameter_secure_keyUpdate
--- PASS: TestAccAWSSSMParameter_secure_keyUpdate (95.38s)
PASS
ok      github.com/terraform-providers/terraform-provider-aws/aws       384.312s

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