Add MYSQL_CONF env var to mariadb, same as mysql, allowing mariadb plugin users to easily override things (#2526)

## Summary

It appears MYSQL_CONF was added to mysql and not mariadb. This patch
gets mariadb in sync with mysql, and also adds a commented-out `#port =
3306` to the template `my.cnf` since that will be the most commonly
needed tweak.

## How was it tested?

Verified that `devbox.d/mariadb/my.cnf` gets symlinked into the
virtualenv and that I can change mariadbd's port with it.

---------

Co-authored-by: John Lago <750845+Lagoja@users.noreply.github.com>

Author: jefft

plugins/mariadb.json

@@ -7,7 +7,8 @@
     "MYSQL_HOME": "{{ .Virtenv }}/run",
     "MYSQL_DATADIR": "{{ .Virtenv }}/data",
     "MYSQL_UNIX_PORT": "{{ .Virtenv }}/run/mysql.sock",
-    "MYSQL_PID_FILE": "{{ .Virtenv }}/run/mysql.pid"
+    "MYSQL_PID_FILE": "{{ .Virtenv }}/run/mysql.pid",
+    "MYSQL_CONF": "{{ .DevboxDir }}/my.cnf"
   },
   "create_files": {
     "{{ .Virtenv }}/run": "",

plugins/mariadb/my.cnf

@@ -1,4 +1,6 @@
 # MySQL configuration file
 
-[mariadb]
+[mariadbd]
+# Change this port if 3306 is already used
+#port = 3306
 log_error=mysql.log

plugins/mysql/my.cnf

@@ -2,3 +2,5 @@
 
 # [mysqld]
 # skip-log-bin
+# Change this port if 3306 is already used
+#port = 3306