Adding new resource for password policy

[petems]

• https://www.vaultproject.io/docs/concepts/password-policies

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Closes #926

Release note for CHANGELOG:

* Adds vault_password_policy resource (closes #926) 

Output from acceptance testing:

$ make testacc TESTARGS='-run=TestAccXXX'
make testacc TESTARGS='-run=TestAccPasswordPolicy'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run=TestAccPasswordPolicy -timeout 120m
?   	github.com/hashicorp/terraform-provider-vault	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/cmd/coverage	[no test files]
?   	github.com/hashicorp/terraform-provider-vault/cmd/generate	[no test files]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/codegen	0.088s [no tests to run]
?   	github.com/hashicorp/terraform-provider-vault/generated	[no test files]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/datasources/transform/decode	1.156s [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/datasources/transform/encode	1.493s [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/alphabet	2.697s [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/role	0.860s [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/template	0.389s [no tests to run]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/generated/resources/transform/transformation	2.136s [no tests to run]
?   	github.com/hashicorp/terraform-provider-vault/schema	[no test files]
testing: warning: no tests to run
PASS
ok  	github.com/hashicorp/terraform-provider-vault/util	0.145s [no tests to run]
=== RUN   TestAccPasswordPolicy
--- PASS: TestAccPasswordPolicy (0.13s)
PASS
ok  	github.com/hashicorp/terraform-provider-vault/vault	2.212s
...

[petems]

@jasonodonnell Mind re-reviewing now? 😄

[jasonodonnell]

LGTM, thanks!