Added support for volume_prototypes on ibm_is_instance resource

[ujjwal-ibm]

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Relates OR Closes #0000

Output from acceptance testing:

% make testacc TEST=./ibm/service/vpc TESTARGS='-run=TestAccIBMISInstance_volprototypes'
=== RUN   TestAccIBMISInstance_volprototypes
--- PASS: TestAccIBMISInstance_volprototypes (310.98s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     312.985s

[ujjwal-ibm]

% make testacc TEST=./ibm/service/vpc TESTARGS='-run=TestAccIBMISInstance_volprototypes'
=== RUN   TestAccIBMISInstance_volprototypes
--- PASS: TestAccIBMISInstance_volprototypes (377.61s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     379.327s

[ujjwal-ibm]

% make testacc TEST=./ibm/service/vpc TESTARGS='-run=TestAccIBMISInstance_basic'                            
=== RUN   TestAccIBMISInstance_basic
--- PASS: TestAccIBMISInstance_basic (255.09s)
=== RUN   TestAccIBMISInstance_basicwithipv4
--- PASS: TestAccIBMISInstance_basicwithipv4 (145.29s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     402.563s