remove unused PIVPN_SERVER from compose env

[FrancYescO]

better awk match for PIVPN_SERVER CN var better handling of HTTPS vars default PIVPN_SERVER to server.conf

[karabelnikov]

PIVPN_SERVER=${PIVPN_SERVER} and PIVPN_CONF=${PIVPN_CONF} These are required variables for openvpn-admin-plus to work. Also, the server.conf file may have a different name for others, depending on their scenarios. It is better to leave it as a variable rather than a static value for all kinds of scenarios.

[FrancYescO]

For conf I've just set a default value (like username and password to have a congruence in default values) and not removed the normal function when setted, for PIVPN_CONF value is already not used was just leaved in documentation, it uses some awk magic to match the value (that I've improved in this PR)