Certmaker should check several locations to find out what the default path is for ca.{crt,key}

[JonTheNiceGuy]

Currently, the path to the ca.key and ca.crt is hardcoded (unless overriden) to /root/nebula. This should have levels of checking, as follows (highest precidence at the top):

• CLI switch (-c, --cert_path)
• Environment Variable (NEBULA_CA_PATH)
• ./
• ~/nebula_ca
• $(eval /etc/defaults/nebula_cert_maker)