Add gcepd support for VolumeInspectByName

[codenrhoden]

Implement VolumeInspectByName for the gcepd driver. This provides an optimized lookup when callers are wanting to lookup a volume by name instead of by ID. This prevents the framework (>= 0.6.1) or the integration driver (<= v0.6.0 and before from having to a Volumes() call and then string match on the name.

Just for fun, some simple performance numbers. This is using REX-Ray 0.9.0, 0.9.1, and 0.9.1+this patch on GCE with 10 volumes defined. Ran a loop 10 times with docker inspect. REX-Ray 0.9 would have called Volumes(), retrieving a list of all volumes and then finding the desired one to inspect. 0.9.1 Would call VolumeInspect() with a flag that has the framework do the Volumes() call followed by the string match instead of the integration driver (eliminating an entire API call between client and server, but it still occurs from the framework so there isn't a big different when doing all-in-one), and this patch would have the framework call VolumeInsepectByName() directly, eliminating the call to Volumes() entirely.

0.9.1+	0.9.1	0.9.0
real 0m0.211s	real 0m0.296s	real 0m0.281s
real 0m0.233s	real 0m0.386s	real 0m0.253s
real 0m0.202s	real 0m0.293s	real 0m0.288s
real 0m0.234s	real 0m0.269s	real 0m0.270s
real 0m0.204s	real 0m0.286s	real 0m0.278s
real 0m0.190s	real 0m0.313s	real 0m0.281s
real 0m0.203s	real 0m0.284s	real 0m0.273s
real 0m0.199s	real 0m0.351s	real 0m0.314s
real 0m0.356s	real 0m0.341s	real 0m0.319s
real 0m0.245s	real 0m0.294s	real 0m0.300s

This results in an average (in seconds) of:

0.9.1+	0.9.1	0.9.0
0.2277	0.3113	0.2857

As expected, there isn't much difference between 0.9.0 and 0.9.1 in this setup. Though I think 0.9.1 would win out if you scaled up the number of volumes and made it a distributed setup. But this patch has the biggest effect overall, because no call to Volumes() is needed at all. DIsparity between the two will only grow as the number of volumes increases.

FWIW, this performance improvement is only realizable by Docker and docker volume commands right now. Some changes would have to occur in REX-Ray to take advantage of this from the rexray CLI.

[codecov-io]

Codecov Report

Merging #579 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #579   +/-   ##
=======================================
  Coverage   28.42%   28.42%           
=======================================
  Files          34       34           
  Lines        2016     2016           
=======================================
  Hits          573      573           
  Misses       1381     1381           
  Partials       62       62

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