Change --testnet flag to --network (#1751)

## Issue Addressed

- Resolves #1689

## Proposed Changes

TBC

## Additional Info

NA

validator_client/src/cli.rs

@@ -47,7 +47,7 @@ pub fn cli_app<'a, 'b>() -> App<'a, 'b> {
                     "The directory which contains the password to unlock the validator \
                     voting keypairs. Each password should be contained in a file where the \
                     name is the 0x-prefixed hex representation of the validators voting public \
-                    key. Defaults to ~/.lighthouse/{testnet}/secrets.",
+                    key. Defaults to ~/.lighthouse/{network}/secrets.",
                 )
                 .takes_value(true)
                 .conflicts_with("datadir")

validator_client/src/lib.rs

@@ -188,13 +188,13 @@ impl<T: EthSpec> ProductionValidatorClient<T> {
         let beacon_node_spec = yaml_config.apply_to_chain_spec::<T>(&T::default_spec())
             .ok_or_else(||
                     "The minimal/mainnet spec type of the beacon node does not match the validator client. \
-                    See the --testnet command.".to_string()
+                    See the --network command.".to_string()
             )?;
 
         if context.eth2_config.spec != beacon_node_spec {
             return Err(
                 "The beacon node is using a different Eth2 specification to this validator client. \
-                See the --testnet command."
+                See the --network command."
                     .to_string(),
             );
         }