[snap] use Ubuntu's OpenSSL packages

[Saviq]

Rather than building and bundling BoringSSL, the snap could use the packages available from the Ubuntu archive.

This would, at least, get us a better security story and ability to build on all arches Ubuntu support (BoringSSL doesn't support big-endian at the moment).

[Saviq]

<xnox> Saviq:  grpc seems to be able to find and use regular OpenSSL if one sets gRPC_SSL_PROVIDER to "package"

One note, though, that we're also using BoringSSL to generate our certificates, but that should be easily switched, too.