Feature/optionally support groups for hosts

[lrsmith]

Based on and supercedes (PR-8)

This PR includes casery232's fix for groups, when managing resources_icinga2_host object. It has been modified to make groups optional rather than required and govend in the correct version of go-icinga2-api, to support optionally delcaring groups, with hosts.

[lrsmith]

Acceptance Test Output

==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v  -timeout 120m
?       github.com/terraform-providers/terraform-provider-icinga2   [no test files]
=== RUN   TestProvider
--- PASS: TestProvider (0.00s)
=== RUN   TestProviderImpl
--- PASS: TestProviderImpl (0.00s)
=== RUN   TestAccCreateCheckcommand
--- PASS: TestAccCreateCheckcommand (0.54s)
=== RUN   TestAccCreateBasicHost
--- PASS: TestAccCreateBasicHost (0.47s)
=== RUN   TestAccCreateGroupHost
--- PASS: TestAccCreateGroupHost (0.35s)
=== RUN   TestAccCreateVariableHost
--- PASS: TestAccCreateVariableHost (0.42s)
=== RUN   TestAccCreateTemplateHost
--- PASS: TestAccCreateTemplateHost (0.46s)
=== RUN   TestAccCreateBasicHostGroup
--- PASS: TestAccCreateBasicHostGroup (0.60s)
=== RUN   TestAccCreateService
--- PASS: TestAccCreateService (0.62s)
PASS
ok      github.com/terraform-providers/terraform-provider-icinga2/icinga2   (cached)```