Nodepool depend on pyzmq which requires a C and C++ compiler chain.
Nodepool also has a transitive dep on cryptography which depends on
libffi-dev being available. Finally cryptography also depends on
libssl-dev for its building. Install build-essential for the compiler
chain and libffi-dev/libssl-dev to make cryptography work prior to
installing nodepool.
Change-Id: If32f6fd868ea9fefca2df610e9a2cb7a06a78631
920cb5e318