Load env-vars for configs

[mostafa]

In this PR I added the feature to load env-vars to override the global configurations. Currently the flattened list of config variables are as below:

clients.default.network
clients.default.receiveBufferSize
clients.default.receiveChunkSize
clients.default.tcpKeepAlivePeriod
loggers.default.fileName
loggers.default.level
loggers.default.noColor
loggers.default.output
loggers.default.permission
loggers.default.timeFormat
pools.default.client
pools.default.size
proxy.default.elastic
proxy.default.elasticClient
proxy.default.logger
proxy.default.pool
proxy.default.reuseElasticClients
server.address
server.enableTicker
server.hardLimit
server.loadBalancer
server.lockOSThread
server.logger
server.multiCore
server.network
server.proxy
server.readBufferCap
server.reuseAddress
server.reusePort
server.socketRecvBuffer
server.socketSendBuffer
server.softLimit
server.tcpKeepAlive
server.tcpNoDelay
server.tickInterval
server.writeBufferCap

And their equivalent env-vars are:

GATEWAYD_CLIENTS_DEFAULT_ADDRESS
GATEWAYD_CLIENTS_DEFAULT_NETWORK
GATEWAYD_CLIENTS_DEFAULT_RECEIVEBUFFERSIZE
GATEWAYD_CLIENTS_DEFAULT_RECEIVECHUNKSIZE
GATEWAYD_CLIENTS_DEFAULT_TCPKEEPALIVEPERIOD
GATEWAYD_LOGGERS_DEFAULT_FILENAME
GATEWAYD_LOGGERS_DEFAULT_LEVEL
GATEWAYD_LOGGERS_DEFAULT_NOCOLOR
GATEWAYD_LOGGERS_DEFAULT_OUTPUT
GATEWAYD_LOGGERS_DEFAULT_PERMISSION
GATEWAYD_LOGGERS_DEFAULT_TIMEFORMAT
GATEWAYD_POOLS_DEFAULT_CLIENT
GATEWAYD_POOLS_DEFAULT_SIZE
GATEWAYD_PROXY_DEFAULT_ELASTIC
GATEWAYD_PROXY_DEFAULT_ELASTICCLIENT
GATEWAYD_PROXY_DEFAULT_LOGGER
GATEWAYD_PROXY_DEFAULT_POOL
GATEWAYD_PROXY_DEFAULT_REUSEELASTICCLIENTS
GATEWAYD_SERVER_ADDRESS
GATEWAYD_SERVER_ENABLETICKER
GATEWAYD_SERVER_HARDLIMIT
GATEWAYD_SERVER_LOADBALANCER
GATEWAYD_SERVER_LOCKOSTHREAD
GATEWAYD_SERVER_LOGGER
GATEWAYD_SERVER_MULTICORE
GATEWAYD_SERVER_NETWORK
GATEWAYD_SERVER_PROXY
GATEWAYD_SERVER_READBUFFERCAP
GATEWAYD_SERVER_REUSEADDRESS
GATEWAYD_SERVER_REUSEPORT
GATEWAYD_SERVER_SOCKETRECVBUFFER
GATEWAYD_SERVER_SOCKETSENDBUFFER
GATEWAYD_SERVER_SOFTLIMIT
GATEWAYD_SERVER_TCPKEEPALIVE
GATEWAYD_SERVER_TCPNODELAY
GATEWAYD_SERVER_TICKINTERVAL
GATEWAYD_SERVER_WRITEBUFFERCAP

Closes #85.