resource(volume): fix parsing mount_flags

[pop]

This fixes a bug where mount_flags were being silently dropped during resoruce creation.

I'm using Nomad v1.3.1 and terraform-provider-nomad v1.4.16.
I noticed that my nomad_volume mount_options.mount_flags weren't being set even though my fs_type was being set.
When creating the same volume directly with the nomad CLI mount_flags were being set so I knew it was a problem with the provider.

When the original error is not being ignored it causes a panic like so:

Stack trace from the terraform-provider-nomad_v1.4.16+dev plugin:

panic: interface conversion: interface {} is []interface {}, not []string

goroutine 55 [running]:
github.com/hashicorp/terraform-provider-nomad/nomad.resourceVolumeCreate(0xc00057f030, 0x1cbd860, 0xc00068c0c0, 0x24, 0x25a8540)
        /Users/pop/Projects/src/github.com/hashicorp/terraform-provider-nomad/nomad/resource_volume.go:353 +0xf77
github.com/hashicorp/terraform-plugin-sdk/helper/schema.(*Resource).Apply(0xc0003ec000, 0xc000612c30, 0xc00077d400, 0x1cbd860, 0xc00068c0c0, 0x1c10501, 0xc00084f8d0, 0xc00081e3f0)
        /Users/pop/go/1.16.2/pkg/mod/github.com/hashicorp/terraform-plugin-sdk@v1.17.2/helper/schema/resource.go:326 +0x273
github.com/hashicorp/terraform-plugin-sdk/helper/schema.(*Provider).Apply(0xc000316b80, 0xc00084fa38, 0xc000612c30, 0xc00077d400, 0xc000473288, 0xc000033170, 0x1c12820)
        /Users/pop/go/1.16.2/pkg/mod/github.com/hashicorp/terraform-plugin-sdk@v1.17.2/helper/schema/provider.go:294 +0x99
github.com/hashicorp/terraform-plugin-sdk/internal/helper/plugin.(*GRPCProviderServer).ApplyResourceChange(0xc000308890, 0x1f68090, 0xc0005d41b0, 0xc00057e1c0, 0xc000308890, 0xc0005d41b0, 0xc000860ba0)
        /Users/pop/go/1.16.2/pkg/mod/github.com/hashicorp/terraform-plugin-sdk@v1.17.2/internal/helper/plugin/grpc_provider.go:895 +0x8a5
github.com/hashicorp/terraform-plugin-sdk/internal/tfplugin5._Provider_ApplyResourceChange_Handler(0x1d1caa0, 0xc000308890, 0x1f68090, 0xc0005d41b0, 0xc0000ba6c0, 0x0, 0x1f68090, 0xc0005d41b0, 0xc0000f9800, 0x733)
        /Users/pop/go/1.16.2/pkg/mod/github.com/hashicorp/terraform-plugin-sdk@v1.17.2/internal/tfplugin5/tfplugin5.pb.go:3305 +0x214
google.golang.org/grpc.(*Server).processUnaryRPC(0xc000338000, 0x1f729b8, 0xc000626300, 0xc0000cc100, 0xc00032a510, 0x25670c0, 0x0, 0x0, 0x0)
        /Users/pop/go/1.16.2/pkg/mod/google.golang.org/grpc@v1.32.0/server.go:1194 +0x52b
google.golang.org/grpc.(*Server).handleStream(0xc000338000, 0x1f729b8, 0xc000626300, 0xc0000cc100, 0x0)
        /Users/pop/go/1.16.2/pkg/mod/google.golang.org/grpc@v1.32.0/server.go:1517 +0xd0c
google.golang.org/grpc.(*Server).serveStreams.func1.2(0xc0000c6240, 0xc000338000, 0x1f729b8, 0xc000626300, 0xc0000cc100)
        /Users/pop/go/1.16.2/pkg/mod/google.golang.org/grpc@v1.32.0/server.go:859 +0xab
created by google.golang.org/grpc.(*Server).serveStreams.func1
        /Users/pop/go/1.16.2/pkg/mod/google.golang.org/grpc@v1.32.0/server.go:857 +0x1fd

I have tested this fix for me and it resolve the issue, but I didn't have time to write automated tests to catch future regressions.

Code feedback is more than welcome.

[pop]

I guess this refs #266 and #260 but my code looks a little different and I didn't include a CHANGELOG item. Happy to add those if y'all want me to.

Note: this is not a dupe of those. Those fix the external_volume resource not the volume resource.

[lgfa29]

Thanks for the fix @pop!

I tried adding tests like I did for #266 but it didn't quite work as I couldn't figure out a way to get the external_id. I think this would be easier with a real CSI cloud volume, but that would take a while to setup.

I tested it manually and does fix the problem 👍