Fix external_log_file functionality and explicitly set Mesos Windows slave hostname

[ionutbalutoiu]

This pull request introduces the following updates:

• File used for --external_log_file must exist. If the file doesn't exist, the following error is given:

Log file created at: 2018/01/16 10:37:02
Running on machine: 15580ACS000000
E0116 10:37:02.354581  1372 slave.cpp:1009] Failed to attach 'C:\DCOS\mesos\log\mesos-service.err.log' to virtual path '/slave/log': Failed to get realpath of 'C:\DCOS\mesos\log\mesos-service.err.log': Failed to get attributes for file 'C:\DCOS\mesos\log\mesos-service.err.log': The system cannot find the file specified.

• Set hostname explicitly for the Windows Mesos slave. If not set, the default hostname advertised to master nodes is the same one for all the Windows slaves:

$ dcos node
                             HOSTNAME                                     IP                          ID                    TYPE
22803acs000001.m1u5b2dryuhutcpk51iv1dm4pb.cx.internal.cloudapp.net     10.0.0.5    c9267e15-1e50-485d-a7ca-745bd8a87bf2-S0  agent
22803acs000001.m1u5b2dryuhutcpk51iv1dm4pb.cx.internal.cloudapp.net     10.1.0.5    c9267e15-1e50-485d-a7ca-745bd8a87bf2-S2  agent
                          master.mesos.                             192.168.255.5    e5b4afb5-42f1-46b0-9427-154da5490efa   master (leader)

$ nslookup 22803acs000001.m1u5b2dryuhutcpk51iv1dm4pb.cx.internal.cloudapp.net
Server:         198.51.100.1
Address:        198.51.100.1#53

Name:   22803acs000001.m1u5b2dryuhutcpk51iv1dm4pb.cx.internal.cloudapp.net
Address: 10.1.0.5

This is wrong and we now set the IP address as the hostname, just as it's happening with the Linux agents.

[ionutbalutoiu]

@yakman2020, Please review and merge if everything is fine. This patch is tested already.

[ionutbalutoiu]

@paulcallen, @yakman2020 Any of you please approve this pull request and we can merge it. It includes fixes for the current upstream DCOS init scripts.