Add users datasource

[jorcau]

Add new gitlab_users datasource that returns a list of users.

It takes optional arguments such as order_by, sort and search, full list in the included documentation.

Examples:

• Get all users

data "gitlab_users" "all_users" {
}

• Get users created before the 5th of April 2018 ordered by username in ascendant order

data "gitlab_users" "username" {
    order_by = "username"
    sort = "asc"
    created_before = "2018-04-05"
}

[pdecat]

Cannot the extern_provider field be also exposed?

[jorcau]

There were actually a lot of missing properties. I added them all, including extern_provider that is named provider to match the API.

[pdecat]

Some formatting nitpicks.

[jorcau]

Some formatting nitpicks.

Thanks for reviewing @pdecat! Didn't see there were tabs instead of spaces :)

[pdecat]

LGTM!

What about acceptance tests results?

[jorcau]

Thanks @pdecat! Here's the acceptance test result:

➜ make testacc TEST=./gitlab TESTARGS='-run=TestAccDataSourceGitlabUsers_*'
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test ./gitlab -v -run=TestAccDataSourceGitlabUsers_* -timeout 120m
=== RUN   TestAccDataSourceGitlabUsers_basic
--- PASS: TestAccDataSourceGitlabUsers_basic (3.52s)
PASS
ok  	github.com/terraform-providers/terraform-provider-gitlab/gitlab	3.543s

[roidelapluie]

Thanks!!