util: modify GetMonsAndClusterID() to take clusterID instead of options

This commit:
- modifies GetMonsAndClusterID() to take clusterID instead of options.
- moves out validation of clusterID is set or not out of GetMonsAndClusterID().
- defines ErrClusterIDNotSet new error for reusability.
- add GetClusterID() to obtain clusterID from options.

Signed-off-by: Rakshith R <rar@redhat.com>

internal/util/errors.go

@@ -33,6 +33,8 @@ var (
 	ErrSnapNameConflict = errors.New("snapshot name conflict")
 	// ErrPoolNotFound is returned when pool is not found.
 	ErrPoolNotFound = errors.New("pool not found")
+	// ErrClusterIDNotSet is returned when cluster id is not set.
+	ErrClusterIDNotSet = errors.New("clusterID must be set")
 )
 
 type errorPair struct {