Fix set IP block visibility on READ if NSX version too low

[wsquan171]

This change fixes setting policy IP block visibility on READ if NSX is below 4.2, where the value will always be nil. This will cause tf to always show diff if the resource spec has non-empty value.

Closes #1227

[ksamoray]

LGTM, would like @annakhm to have a look as well as this behavior is not consistent throughout the provider code. Maybe we could at least warn that this attribute has been ignored?

[annakhm]

What happens in NSX 4.2 if this setting is unspecified? Does NSX send back a default?

[wsquan171]

@annakhm this param is unknown to NSX < 4.2. On read it will be empty value, and if included on write it will error.

[annakhm]

@annakhm this param is unknown to NSX < 4.2. On read it will be empty value, and if included on write it will error.

My question was for NSX 4.2. This param is optional, so user can skip it. However, for such enumerations NSX would normally send back a default (not always though). I wanted to verify we don't need to set this param as Computed for NSX 4.2, regardless of earlier version

[wsquan171]

I wanted to verify we don't need to set this param as Computed for NSX 4.2, regardless of earlier version

For NSX 4.2, if the value is unset, NSX will send back as-is (also unset). This doesn't need to be computed.