use ReinstallInstanceRequest instead of NewReinstallInstanceRequestWithDefaults

[m0build-io]

This commit fix the issue where the default_user would be reset to admin on reinstallation independently from the value set in terrafom see issue #45.
The problem is that the NewReinstallInstanceRequestWithDefaults return a ReinstallInstanceRequest with a DefaultUser value of "admin". Because of this, on reinstallation, the default_user would unexpectedly be set to admin.

For example:

1. Define a contabo_resource with debian 10 and default_user=root using contabo Web UI (I don't use terraform here to not raise the issue immediately

2. Check using cntb that it is installed correctly

cntb get instances --oauth2-clientid="" --oauth2-client-secret="" --oauth2-user="" --oauth2-password=""
INSTANCEID  NAME          DISPLAYNAME   STATUS     IMAGEID                               REGION  PRODUCTID  IPV4     IPV6                                     DEFAULTUSER
123456789   vmi123456789  display_name  installing bcaaa769-2a1d-4b54-b93b-e329360457e1  EU      V1         1.2.3.4  1234:1234:1234:1234:0000:0000:0000:0001  root

3. Once installed. Trigger a reinstallation using terrafom without changing the default_user, only changing the image:

resource "contabo_instance" "instance" {
        default_user = "root"
        image_id = "4efbc0ba-2313-4fe1-842a-516f8652e729"
}

4. Check using cntb, the default_user has been reset to "admin" :(

INSTANCEID  NAME          DISPLAYNAME    STATUS     IMAGEID                               REGION  PRODUCTID  IPV4     IPV6                                     DEFAULTUSER
123456789   vmi123456789  display_name   installing 4efbc0ba-2313-4fe1-842a-516f8652e729  EU      V1         1.2.3.4  1234:1234:1234:1234:0000:0000:0000:0001  admin