Feature/support proxy for freshclam

[asteel-gsa]

PR for #19

Test Case (with ajilaag/clamav-rest image):

/etc/clamav # ./test.sh 
Replaced proxy server #=> ensuring sed works with new delimiter when testing
Replaced proxy port #=> ensuring sed works with new delimiter when testing
Replaced proxy user and pass #=> ensuring sed works with new delimiter when testing

/etc/clamav # cat freshclam.conf | grep "HTTP*"
# The HTTPProxyServer may be prefixed with [scheme]:// to specify which kind
#   http://     HTTP Proxy. Default when no scheme or proxy type is specified.
#   https://    HTTPS Proxy. (Added in 7.52.0 for OpenSSL, GnuTLS and NSS)
HTTPProxyServer https://proxy-test.com
HTTPProxyPort 54321
HTTPProxyUsername TESTUSER
HTTPProxyPassword TEST/PASS
#HTTPUserAgent SomeUserAgentIdString

Usage:
If $PROXY_SERVER is not set in an environment variable, then none of this will run
If $PROXY_SERVER is set then replace the value in freshclam.conf

• Since port, username and password are not required, only apply them to freshclam.conf if $PROXY_PORT, $PROXY_USERNAME and $PROXY_PASSWORD are set in an environment variable

[mogul]

@rhermanek would you mind merging this for us? We're hoping to avoid maintaining our own image.