provider/alicloud: Add the function of replacing ecs instance's system disk

[xiaozhu36]

This PR adds a function that can replace system disk for an existing ecs instance. And while replacing system disk, it can chang system disk size.

Run the PR's testcase result as follows:

TF_ACC=1 go test ./alicloud -v -run=TestAccAlicloudInstance -timeout 120m
=== RUN TestAccAlicloudInstanceTypesDataSource_basic
--- PASS: TestAccAlicloudInstanceTypesDataSource_basic (14.70s)
=== RUN TestAccAlicloudInstance_basic
--- PASS: TestAccAlicloudInstance_basic (104.20s)
=== RUN TestAccAlicloudInstance_vpc
--- PASS: TestAccAlicloudInstance_vpc (112.01s)
=== RUN TestAccAlicloudInstance_userData
--- PASS: TestAccAlicloudInstance_userData (125.50s)
=== RUN TestAccAlicloudInstance_multipleRegions
--- PASS: TestAccAlicloudInstance_multipleRegions (342.78s)
=== RUN TestAccAlicloudInstance_multiSecurityGroup
--- PASS: TestAccAlicloudInstance_multiSecurityGroup (3.44s)
=== RUN TestAccAlicloudInstance_multiSecurityGroupByCount
--- PASS: TestAccAlicloudInstance_multiSecurityGroupByCount (3.72s)
=== RUN TestAccAlicloudInstance_NetworkInstanceSecurityGroups
--- PASS: TestAccAlicloudInstance_NetworkInstanceSecurityGroups (107.63s)
=== RUN TestAccAlicloudInstance_tags
--- PASS: TestAccAlicloudInstance_tags (91.18s)
=== RUN TestAccAlicloudInstance_update
--- PASS: TestAccAlicloudInstance_update (87.35s)
=== RUN TestAccAlicloudInstanceImage_update
--- PASS: TestAccAlicloudInstanceImage_update (212.84s)
=== RUN TestAccAlicloudInstance_privateIP
--- PASS: TestAccAlicloudInstance_privateIP (99.39s)
=== RUN TestAccAlicloudInstance_associatePublicIP
--- PASS: TestAccAlicloudInstance_associatePublicIP (113.35s)
=== RUN TestAccAlicloudInstance_vpcRule
--- PASS: TestAccAlicloudInstance_vpcRule (118.18s)
PASS
ok github.com/alibaba/terraform-provider/alicloud 118.200s

[stack72]

This LGTM! Thanks for this :)

% make testacc TEST=./builtin/providers/alicloud TESTARGS='-run=TestAccAlicloudInstance'                                                     ✭
==> Checking that code complies with gofmt requirements...
go generate $(go list ./... | grep -v /terraform/vendor/)
2017/06/05 00:11:02 Generated command/internal_plugin_list.go
TF_ACC=1 go test ./builtin/providers/alicloud -v -run=TestAccAlicloudInstance -timeout 120m
=== RUN   TestAccAlicloudInstanceTypesDataSource_basic
--- PASS: TestAccAlicloudInstanceTypesDataSource_basic (202.13s)
=== RUN   TestAccAlicloudInstance_basic
--- PASS: TestAccAlicloudInstance_basic (335.21s)
=== RUN   TestAccAlicloudInstance_vpc
--- PASS: TestAccAlicloudInstance_vpc (328.67s)
=== RUN   TestAccAlicloudInstance_userData
--- PASS: TestAccAlicloudInstance_userData (346.38s)
=== RUN   TestAccAlicloudInstance_multipleRegions
--- PASS: TestAccAlicloudInstance_multipleRegions (286.93s)
=== RUN   TestAccAlicloudInstance_multiSecurityGroup
--- PASS: TestAccAlicloudInstance_multiSecurityGroup (631.92s)
=== RUN   TestAccAlicloudInstance_multiSecurityGroupByCount
--- PASS: TestAccAlicloudInstance_multiSecurityGroupByCount (276.32s)
=== RUN   TestAccAlicloudInstance_NetworkInstanceSecurityGroups
--- PASS: TestAccAlicloudInstance_NetworkInstanceSecurityGroups (297.05s)
=== RUN   TestAccAlicloudInstance_tags
--- PASS: TestAccAlicloudInstance_tags (318.00s)
=== RUN   TestAccAlicloudInstance_update
--- PASS: TestAccAlicloudInstance_update (273.76s)
=== RUN   TestAccAlicloudInstance_privateIP
--- PASS: TestAccAlicloudInstance_privateIP (362.18s)
=== RUN   TestAccAlicloudInstance_associatePublicIP
--- PASS: TestAccAlicloudInstance_associatePublicIP (407.83s)
=== RUN   TestAccAlicloudInstance_vpcRule
--- PASS: TestAccAlicloudInstance_vpcRule (437.96s)
PASS
ok	github.com/hashicorp/terraform/builtin/providers/alicloud	4648.547s

[ghost]

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