Fix resource egress rule to accept protocol type ALL

[harikrishna-patnala]

This PR fixes #3

Previously only protocol types icmp, tcp, udp are supported, with this fix we have added support for protocol type ALL.

Tested by following steps

1. Created a egress rule using below config and "terraform apply" created the new egress rule
   resource "cloudstack_egress_firewall" "default" {
   network_id = "2047d8a9-789d-41e6-8eba-a7fa597d0c92"
   rule {
   protocol = "ALL"
   }
2. Removed the above config from configuration file and "terraform apply" deleted the above created rule

[rohityadavcloud]

This needs testing, how was it tested @harikrishna-patnala (could be maybe integrate .travis.yml to do something?)

[harikrishna-patnala]

I've tested this PR manually doing crud operations on resource egress firewall with both protocol types "ALL" and "icmp".

Yes adding .travis.yml for the basic testing for all other PRs is a good idea. I'll add it as a different PR.