fix(mysql): improve random(with LCG) password for mysql user

[avinzheng]

fix: #511

[coveralls]

Coverage increased (+0.1%) to 81.702% when pulling c4bc2a1 on imzhengfei:master into 53a3425 on TryGhost:master.

[avinzheng]

Use Linear Congruential Generator to generate a random number between 0 and 1, in stead of Math.random().

[coveralls]

Coverage increased (+0.2%) to 81.747% when pulling 9717fde on imzhengfei:master into 53a3425 on TryGhost:master.

[avinzheng]

fixed a spelling error

[coveralls]

Coverage increased (+0.2%) to 81.747% when pulling f8dcf0d on imzhengfei:master into 53a3425 on TryGhost:master.

[acburdine]

@imzhengfei sorry for the delay on this! would it be possible to move the generateRandomPassword function to a method on the nginx extension class? keeps things a bit cleaner :)

[avinzheng]

@acburdine I am not sure, do you mean MySQLExtension class ?

[acburdine]

@imzhengfei my apologies, yes that's what I meant.

However, as I commented here, there's another issue in that the initial create user command is what actually is triggering the insecure password error - so we need to find a way around that as well.

[acburdine]

Closing in favor of #546 -> ultimately it required other changes than this. @gompa's approach in #534 was a bit better than this I think too - it was less code to maintain + the library used in that PR uses crypto.randomBytes rather than something based off of a technically predictable seed.