Add consistent variable for "--verbose --help" arguments

[tianon]

Given these are duplicated (or should be -- as seen in the diff we already messed this up!) it makes sense to make them share a common variable.

This is ported from the changes I've proposed in MariaDB/mariadb-docker#340 (where MariaDB needs an additional parameter).

[yosifkit]

For my (future) reference on why --encrypt-tmp-files from MariaDB/mariadb-docker#340 was not added here; the flag is not available in MySQL.

$ docker run -it --rm mysql:5.6 --verbose --help | grep encrypt
  --block-encryption-mode=name 
  --des-key-file=name Load keys for des_encrypt() and des_encrypt from given
block-encryption-mode                                      aes-128-ecb
$ docker run -it --rm mysql:5.7 --verbose --help | grep encrypt
  --block-encryption-mode=name 
  --des-key-file=name Load keys for des_encrypt() and des_encrypt from given
block-encryption-mode                                        aes-128-ecb
$ docker run -it --rm mysql:8.0 --verbose --help | grep encrypt
  --binlog-encryption Enable/disable binary and relay logs encryption.
  --binlog-rotate-encryption-master-key-at-startup 
                      Force binlog encryption master key rotation at startup
  --block-encryption-mode=name 
  --default-table-encryption 
                      encryption property unless the user specifies an explicit
                      encryption property.
  --innodb-redo-log-encrypt 
  --innodb-undo-log-encrypt 
  --table-encryption-privilege-check 
binlog-encryption                                            FALSE
binlog-rotate-encryption-master-key-at-startup               FALSE
block-encryption-mode                                        aes-128-ecb
default-table-encryption                                     FALSE
innodb-redo-log-encrypt                                      FALSE
innodb-undo-log-encrypt                                      FALSE
table-encryption-privilege-check                             FALSE