Creates platform specific config and main modules

[venilnoronha]

Description

This commit separates platform specific config and plugin serving code from the generic one. It will aid to the development of the Windows plugin.

• Extracts unix sock service from main.go into main_linux.go.
• Moves linux specific config from config.go into config_linux.go.
• Renames config_test.go to config_linux_test.go.

Test Results

The test-all target passes locally with the following output (tail'd):

ssh  -kTax -o StrictHostKeyChecking=no root@10.160.205.225 /tmp/docker-volume-vsphere/vmdkops.test -test.v
=== RUN   TestCommands
Detaching loopback device /dev/loop1001
Detaching loopback device /dev/loop1001
Detaching loopback device /dev/loop1001
--- PASS: TestCommands (0.65s)
	cmd_test.go:30: 
		Creating Test Volume with name = [TestVol]...
PASS
coverage: 43.3% of statements
ssh  -kTax -o StrictHostKeyChecking=no root@10.160.205.225 /tmp/docker-volume-vsphere/docker-volume-vsphere.test -test.v \
		-v DefaultTestVol \
		-H1 tcp://10.160.205.225:2375 -H2 tcp://10.160.207.83:2375
=== RUN   TestSanity
2017-06-26T10:45:39-07:00 START: Running TestSanity on  tcp://10.160.205.225:2375 (may take a while)...
2017-06-26T10:45:45-07:00 END: Running TestSanity on  tcp://10.160.205.225:2375 (may take a while)...
--- PASS: TestSanity (6.38s)
	sanity_test.go:201: Successfully connected to tcp://10.160.205.225:2375
	sanity_test.go:201: Successfully connected to tcp://10.160.207.83:2375
	sanity_test.go:219: Creating vol=DefaultTestVol on client tcp://10.160.205.225:2375.
	sanity_test.go:105: Running cmd=&[touch /mnt/testvol/DefaultTestVol/file_to_touch] with vol=DefaultTestVol on client tcp://10.160.205.225:2375
	sanity_test.go:105: Running cmd=&[stat /mnt/testvol/DefaultTestVol/file_to_touch] with vol=DefaultTestVol on client tcp://10.160.205.225:2375
=== RUN   TestConcurrency
2017-06-26T10:45:45-07:00 Running concurrent tests on tcp://10.160.205.225:2375 and tcp://10.160.207.83:2375 (may take a while)...
2017-06-26T10:45:45-07:00 START: Running create/delete multi-host concurrent test ...
2017-06-26T10:45:56-07:00 END: Running create/delete multi-host concurrent test ...
Running same docker host concurrent create/delete test on tcp://10.160.205.225:2375...
2017-06-26T10:46:03-07:00 START: Running clone concurrent test...
2017-06-26T10:46:11-07:00 END: Running clone concurrent test...
--- PASS: TestConcurrency (25.70s)
	sanity_test.go:201: Successfully connected to tcp://10.160.205.225:2375
	sanity_test.go:201: Successfully connected to tcp://10.160.207.83:2375
PASS
coverage: 39.1% of statements

[msterin]

@venilnoronha - when you close an issue/PR, please put a quick one liner about the reason to close.
I saw you open #1482, but i did not understand why

[venilnoronha]

@msterin sure.

I had amended my commit here, but for some reason the PR was still showing the old one. So, I closed this and opened #1482.