Re-add ability to rebuild

[holmesb]

Discussion resulted in agreement to reintroduce the ability to rebuild. This was removed in v6. New foreman_host argument set_build_flag will default to false. This will still allow the Foreman default behaviour of initially building new hosts, but will avoid the previous incarnation's (build) risk of accidental rebuilds after next Terraform run. Expectation of new behaviour vs current:

• Now (v6): no set_build_flag argument exists, initial creation of foreman_host results in it being built, since this is the Foreman default. Re-running Terraform does not rebuild.
• New: set_build_flag = false (default): This provider does not tell Foreman API to disable the build flag. Instead, due to the fact Foreman defaults to build = true, initial creation of foreman_host in Terraform still results in host being built. Re-running of Terraform will not rebuild.
• New: set_build_flag = true: Initial creation of foreman_host results in it being built. Re-run of Terraform will rebuild. Changing back to false will prevent rebuild.

[bitkeks]

Re-run of Terraform will rebuild. Changing back to false will prevent rebuild

Please note that setting the set_build_flag argument in TF to true does not automatically re-install the host through Foreman. The flag will have an effect after the next reboot of the machine, because Foreman will then trigger the installation process again. This workflow is out of scope of Terraform.