How to drive a 555 timer with pwm input?
Often neglected on 555 examples, they have a control voltage pin. It's usually seen simply connected with a capacitor to ground, to reduce noise. However, if you apply a control voltage to it, it varies the period. 

With a single 555 connected as an astable, it will vary the frequency.

With a 556 (dual 555) as an astable followed by a monostable, you can control the duty cycle at constant frequency. I use this arrangement for a stable and easy to build volts to model servo drive.