fix all bits broken by viper API changes (#5982)

github.com/spf13/viper's recent releases introduced a semantic
change in some public API such as viper.IsSet(), which have
broken some of our flags checks. Instead of checking whether
users have changed a flag's default value we should rely on such
defaults and adjust runtime behaviour accordingly. In order to do
so, it's important that we pick sane defaults for all our flags.

The --pruning flag and configuration option now allow for a
fake custom strategy. When users elect custom, then the
pruning-{keep,snapshot}-every options are interpreted and
parsed; else they're ignored.
Zero is pruning-{keep,snapshot}-every default value. When
users choose to set a custom pruning strategy they are
signalling that they want more fine-grainted control, therefore
it's legitimate to expect them to know what they are doing and
enter valid values for both options.

Ref #5964

x/distribution/client/common/common_test.go

@@ -3,14 +3,17 @@ package common
 import (
 	"testing"
 
+	"github.com/spf13/viper"
 	"github.com/stretchr/testify/require"
 
 	"github.com/cosmos/cosmos-sdk/client/context"
+	"github.com/cosmos/cosmos-sdk/client/flags"
 	"github.com/cosmos/cosmos-sdk/codec"
 )
 
 func TestQueryDelegationRewardsAddrValidation(t *testing.T) {
 	cdc := codec.New()
+	viper.Set(flags.FlagOffline, true)
 	ctx := context.NewCLIContext().WithCodec(cdc)
 	type args struct {
 		delAddr string