Replace OpenOptions::new with File::options to be readable (#3059)

## Issue Addressed

Closes #3049 

This PR updates widely but this replace is safe as `File::options()` is equivelent to `OpenOptions::new()`.
ref: https://doc.rust-lang.org/stable/src/std/fs.rs.html#378-380

common/validator_dir/src/builder.rs

@@ -4,7 +4,7 @@ use deposit_contract::{encode_eth1_tx_data, Error as DepositError};
 use eth2_keystore::{Error as KeystoreError, Keystore, KeystoreBuilder, PlainText};
 use filesystem::create_with_600_perms;
 use rand::{distributions::Alphanumeric, Rng};
-use std::fs::{create_dir_all, OpenOptions};
+use std::fs::{create_dir_all, File};
 use std::io::{self, Write};
 use std::path::{Path, PathBuf};
 use types::{ChainSpec, DepositData, Hash256, Keypair, Signature};
@@ -197,7 +197,7 @@ impl<'a> Builder<'a> {
                     return Err(Error::DepositDataAlreadyExists(path));
                 } else {
                     let hex = format!("0x{}", hex::encode(&deposit_data));
-                    OpenOptions::new()
+                    File::options()
                         .write(true)
                         .read(true)
                         .create(true)
@@ -214,7 +214,7 @@ impl<'a> Builder<'a> {
                 if path.exists() {
                     return Err(Error::DepositAmountAlreadyExists(path));
                 } else {
-                    OpenOptions::new()
+                    File::options()
                         .write(true)
                         .read(true)
                         .create(true)
@@ -267,7 +267,7 @@ fn write_keystore_to_file(path: PathBuf, keystore: &Keystore) -> Result<(), Erro
     if path.exists() {
         Err(Error::KeystoreAlreadyExists(path))
     } else {
-        let file = OpenOptions::new()
+        let file = File::options()
             .write(true)
             .read(true)
             .create_new(true)

common/validator_dir/src/validator_dir.rs

@@ -6,7 +6,7 @@ use deposit_contract::decode_eth1_tx_data;
 use derivative::Derivative;
 use eth2_keystore::{Error as KeystoreError, Keystore, PlainText};
 use lockfile::{Lockfile, LockfileError};
-use std::fs::{read, write, OpenOptions};
+use std::fs::{read, write, File};
 use std::io;
 use std::path::{Path, PathBuf};
 use tree_hash::TreeHash;
@@ -211,7 +211,7 @@ pub fn unlock_keypair<P: AsRef<Path>>(
     password_dir: P,
 ) -> Result<Keypair, Error> {
     let keystore = Keystore::from_json_reader(
-        &mut OpenOptions::new()
+        &mut File::options()
             .read(true)
             .create(false)
             .open(keystore_path)
@@ -236,7 +236,7 @@ pub fn unlock_keypair_from_password_path(
     password_path: &Path,
 ) -> Result<Keypair, Error> {
     let keystore = Keystore::from_json_reader(
-        &mut OpenOptions::new()
+        &mut File::options()
             .read(true)
             .create(false)
             .open(keystore_path)