It is given that the expansion of $( a x + b ) ^ { 3 }$ is $8 x ^ { 3 } - p x ^ { 2 } + 18 x - 3 \sqrt { 3 }$, where $a , b$ and $p$ are real constants.
What is the value of $p$ ?
A $- 12 \sqrt { 3 }$
B $- 6 \sqrt { 3 }$
C $- 4 \sqrt { 3 }$
D $- \sqrt { 3 }$
E $\sqrt { 3 }$
F $4 \sqrt { 3 }$
G $6 \sqrt { 3 }$
H $12 \sqrt { 3 }$