Update sr data source to have an optional pool_id attribute

[ddelnano]

This is to address the final feedback on #15. Users that have storage repositories with the same name across pools are unable to use the current storage repository data source. This adds an optional pool_id attribute so that it can be used when necessary.

Acceptance criteria

• [x] make testacc passes

  ddelnano@ddelnano-desktop:~/go/src/github.com/ddelnano/terraform-provider-xenorchestra$ TF_ACC=1 go test ./... -v
  ?       github.com/ddelnano/terraform-provider-xenorchestra     [no test files]
  === RUN   TestGetPIFByDevice
  --- PASS: TestGetPIFByDevice (0.15s)
  === RUN   TestStorageRepositoryCompare
  --- PASS: TestStorageRepositoryCompare (0.00s)
  === RUN   TestGetStorageRepositoryByType
  --- PASS: TestGetStorageRepositoryByType (0.16s)
  === RUN   TestGetTemplate
  --- PASS: TestGetTemplate (0.22s)
  === RUN   TestThatUnmarshalingWorks
  --- PASS: TestThatUnmarshalingWorks (0.00s)
  === RUN   TestUnmarshalingVmObject
  --- PASS: TestUnmarshalingVmObject (0.00s)
  PASS
  ok      github.com/ddelnano/terraform-provider-xenorchestra/client      0.548s
  === RUN   TestAccXenorchestraDataSource_pif
  --- PASS: TestAccXenorchestraDataSource_pif (0.70s)
  === RUN   TestAccXenorchestraDataSource_storageRepository
  --- PASS: TestAccXenorchestraDataSource_storageRepository (0.64s)
  === RUN   TestAccXenorchestraDataSource_storageRepositoryWithPoolId
  --- PASS: TestAccXenorchestraDataSource_storageRepositoryWithPoolId (0.82s)
  === RUN   TestAccXenorchestraDataSource_storageRepositoryWithNonExistantPoolId
  --- PASS: TestAccXenorchestraDataSource_storageRepositoryWithNonExistantPoolId (0.64s)
  === RUN   TestAccXenorchestraDataSource_template
  --- PASS: TestAccXenorchestraDataSource_template (0.77s)
  === RUN   TestAccXenorchestraCloudConfig_readAfterDelete
  --- PASS: TestAccXenorchestraCloudConfig_readAfterDelete (1.11s)
  === RUN   TestAccXenorchestraCloudConfig_create
  --- PASS: TestAccXenorchestraCloudConfig_create (1.05s)
  === RUN   TestAccXenorchestraCloudConfig_updateName
  --- PASS: TestAccXenorchestraCloudConfig_updateName (1.53s)
  === RUN   TestAccXenorchestraCloudConfig_updateTemplate
  --- PASS: TestAccXenorchestraCloudConfig_updateTemplate (1.55s)
  === RUN   TestAccXenorchestraCloudConfig_import
  --- PASS: TestAccXenorchestraCloudConfig_import (1.08s)
  === RUN   TestAccXenorchestraVm_create
  [DEBUG] VM params map[string]interface {}{"CPUs":1, "VIFs":[]map[string]string{map[string]string{"network":"d225cf00-36f8-e6d6-6a29-02636d4de56b"}}, "bootAfterCreate":true, "cloudConfig":"template", "coreOs":false, "cpuCap":interface {}(nil), "cpuWeight":interface {}(nil), "existingDisks":map[string]interface {}{"0":map[string]interface {}{"$SR":"7f469400-4a2b-5624-cf62-61e522e50ea1", "name_label":"Ubuntu Bionic Beaver 18.04_imavo", "size":10000000000}}, "memoryMax":256000000, "name_description":"description", "name_label":"Name", "template":"2dd0373e-0ed5-7413-a57f-1958d03b698c"}--- PASS: TestAccXenorchestraVm_create (142.30s)
  PASS
  ok      github.com/ddelnano/terraform-provider-xenorchestra/xoa 152.218s