This patch ads an ability to receive TLS connections
to ProxySQL. Certificates and variable lookups are
added in order for TLS to be enabled by
<project_name>_database_internal_tls_enable.
Note that in order for this to work, mysql
connection strings need to have TLS enabled,
which can be added in separate per-service patches
Change-Id: I2c06ce5e138f52259c1725dae37f25c1b00d1e6b