Since the relase of CentOS 7 there is a bug in the preboot crypto, no password request shows up.
Bug report filed but since then multiple releases without a fix – seems to be not important.
The reason is the boot command rhgb quit, Just remove it and you are fine.
edit /etc/default/grub and remove rhgb quiet from GRUB_CMDLINE_LINUX
run grub2-mkconfig -o /boot/grub2/grub.cfg
To boot the first time you can edit the boot command on the start and remove it (it is just untill the next boot so of you want it permanent use the command noticed before)