Convert ssh key ppk to pem

[aizu]

Using Mac

# brew install putty

Then,

# puttygen key.ppk -O private-openssh -o key.pem

Ref: https://servicehub.ucdavis.edu/servicehub?id=ucd_kb_article&sys_id=623e3d4e1b4e9d100e0b2068b04bcb21