Strip quotes when parsing my.cnf

Remove leading/trailing single or double quotes when parsing the my.cnf
file in mysql_user and mysql_db.

Do this so that these modules parse the my.cnf file the same way that
the mysql client does.

From: http://dev.mysql.com/doc/refman/5.0/en/option-files.html

    You can optionally enclose the value within single quotation marks
    or double quotation marks, which is useful if the value contains a
    “#” comment character.

Fixes #2405
Merge pull request #2496 from lorin/mycnf-quotes

Strip quotes when parsing my.cnf