Add DIS objects

[kolsean]

This PR adds 2 objects:

Resources
sbercloud_dis_stream

Data Sources
sbercloud_dis_partition

This closes #68

All acceptance tests are done:

make testacc TEST='./sbercloud' TESTARGS='-run TestAccDis'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./sbercloud -v -run TestAccDis -timeout 360m -parallel=4
=== RUN   TestAccDisPartitionV2DataSource_basic
=== PAUSE TestAccDisPartitionV2DataSource_basic
=== RUN   TestAccDisStreamV2_basic
=== PAUSE TestAccDisStreamV2_basic
=== CONT  TestAccDisPartitionV2DataSource_basic
=== CONT  TestAccDisStreamV2_basic
--- PASS: TestAccDisStreamV2_basic (8.83s)
--- PASS: TestAccDisPartitionV2DataSource_basic (10.59s)
PASS
ok      github.com/sbercloud-terraform/terraform-provider-sbercloud/sbercloud   10.960s