Remove read operation on entity alias update

[benashz]

The identityEntityAliasUpdate function was performing a read operation
when it should not have been. The read operation should be constrained
to the defined ReadContext.

• add new GetAPIRequestData() function that generalizes the translation
  of schema.ResourceData values to their Vault request data equivalents.

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Relates OR Closes #0000

Release note for CHANGELOG:

Output from acceptance testing:

 $ time make testacc TESTARGS='-v -test.run TestAccIdentityEntityAlias*'

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test -v -v -test.run TestAccIdentityEntityAlias* -timeout 30m ./...

ok      github.com/hashicorp/terraform-provider-vault/util      (cached) [no tests to run]
=== RUN   TestAccIdentityEntityAlias
--- PASS: TestAccIdentityEntityAlias (3.14s)
=== RUN   TestAccIdentityEntityAliasDuplicateFlow
--- PASS: TestAccIdentityEntityAliasDuplicateFlow (7.48s)
=== RUN   TestAccIdentityEntityAlias_Update
--- PASS: TestAccIdentityEntityAlias_Update (3.09s)
=== RUN   TestAccIdentityEntityAlias_Metadata
--- PASS: TestAccIdentityEntityAlias_Metadata (3.07s)
PASS
ok      github.com/hashicorp/terraform-provider-vault/vault     19.717s
make testacc TESTARGS='-v -test.run TestAccIdentityEntityAlias*'  42.40s user 18.94s system 229% cpu 26.747 total


...

[vinay-gopalan]

Looks great!