[SNAT/DNAT]: add description/tests

[artem-lifshits]

Acceptance tests

=== RUN   TestSnatRuleLifeCycle
    natgateways_test.go:46: Attempting to create Nat Gateway
    natgateways_test.go:67: Created Nat Gateway: 6469c23e-f6cc-47e7-befe-b4d0ab45ffb8
    helpers.go:17: Attempting to create eip/bandwidth
    helpers.go:33: Waiting for eip cfeba65c-3812-47fe-a42a-8644cd968955 to be active
    helpers.go:40: Created eip/bandwidth: cfeba65c-3812-47fe-a42a-8644cd968955
    snatrules_test.go:21: Attempting to create SNAT rule
    snatrules_test.go:31: Created SNAT rule: fbe8a8ee-3614-4217-840a-889680ebb2c2
    snatrules_test.go:40: Attempting to Obtain SNAT rule: fbe8a8ee-3614-4217-840a-889680ebb2c2
    snatrules_test.go:46: Attempting to Obtain SNAT rules in NAT Gateway: 6469c23e-f6cc-47e7-befe-b4d0ab45ffb8
    snatrules_test.go:34: Attempting to delete SNAT rule: fbe8a8ee-3614-4217-840a-889680ebb2c2
    snatrules_test.go:37: Deleted SNAT rule: fbe8a8ee-3614-4217-840a-889680ebb2c2
    helpers.go:49: Attempting to delete eip/bandwidth: cfeba65c-3812-47fe-a42a-8644cd968955
    helpers.go:55: Waitting for eip cfeba65c-3812-47fe-a42a-8644cd968955 to be deleted
    helpers.go:60: Deleted eip/bandwidth: cfeba65c-3812-47fe-a42a-8644cd968955
    natgateways_test.go:73: Attempting to delete Nat Gateway: 6469c23e-f6cc-47e7-befe-b4d0ab45ffb8
    natgateways_test.go:78: Nat Gateway is deleted: 6469c23e-f6cc-47e7-befe-b4d0ab45ffb8
--- PASS: TestSnatRuleLifeCycle (15.84s)
PASS

Process finished with the exit code 0

=== RUN   TestDnatRuleLifeCycle
    natgateways_test.go:46: Attempting to create Nat Gateway
    natgateways_test.go:67: Created Nat Gateway: 1d90cf5f-c769-4f22-97fb-dda800e5b48a
    helpers.go:17: Attempting to create eip/bandwidth
    helpers.go:33: Waiting for eip 96f984d0-84f2-45a7-a779-921854670806 to be active
    helpers.go:40: Created eip/bandwidth: 96f984d0-84f2-45a7-a779-921854670806
    dnatrulest_test.go:31: Attempting to create DNAT rule, Direct Connect scenario
    dnatrulest_test.go:44: Created DNAT DC rule: 348b9645-684c-4a67-8b76-0d221208fe85
    dnatrulest_test.go:107: Attempting to get DNAT rule: 348b9645-684c-4a67-8b76-0d221208fe85
    dnatrulest_test.go:113: Attempting to list DNAT rules
    dnatrulest_test.go:47: Attempting to delete DC DNAT rule: 348b9645-684c-4a67-8b76-0d221208fe85
    dnatrulest_test.go:50: Deleted DC DNAT rule: 348b9645-684c-4a67-8b76-0d221208fe85
    helpers.go:49: Attempting to delete eip/bandwidth: 96f984d0-84f2-45a7-a779-921854670806
    helpers.go:55: Waitting for eip 96f984d0-84f2-45a7-a779-921854670806 to be deleted
    helpers.go:60: Deleted eip/bandwidth: 96f984d0-84f2-45a7-a779-921854670806
    natgateways_test.go:73: Attempting to delete Nat Gateway: 1d90cf5f-c769-4f22-97fb-dda800e5b48a
    natgateways_test.go:78: Nat Gateway is deleted: 1d90cf5f-c769-4f22-97fb-dda800e5b48a
--- PASS: TestDnatRuleLifeCycle (16.79s)
PASS

Process finished with the exit code 0