Use ECDSA in PKI tests

[markylaing]

The PKI tests use easyrsa for issuing certificates. We may need to use another tool for this if ECDSA is not supported by easyrsa.

At some point, we should move away from RSA and use ECDSA as that's what LXD uses by default nowadays. For another PR, ofc.

Originally posted by @simondeziel in https://github.com/canonical/lxd/pull/13705#discussion_r1672752677