client: even cleaner use of Transport

First off, sorry for the noise. This is a cleaner step of #8508

Found more of a root cause of the open file handles.
After more testing I found that the open file descriptors will still
occur for TCP:// connections to the daemon, causing client and/or daemon
to fail.

The issue was instantiating a new http.Transport on _ever_ client
request. So each instance held the prior connection alive, but was only
ever used once.

By moving it out to the initilization of DockerCli, we can now have
reuse of idled connections. Simplifies the garbage overhead of the
client too, though that's not usually a deal.

Signed-off-by: Vincent Batts <vbatts@redhat.com>
Add info on --device flag permissions ':rwm'

Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au>

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
add BytesSize in pkg/units

Signed-off-by: Victor Vieux <vieux@docker.com>
Replace '%s' in create tests

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Replace '%s' in diff tests

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Replace '%s' in rm tests

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Replace '%s' in run tests

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Replace '%s' in test utils

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Merge branch 'master' into bump_v1.3.0
Bump to version 1.3.0

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
Expanded release notes

Made it clear signed images is a preview feature and added a little more info about how the feature works.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
Merge pull request #8595 from fredlf/signed-image-doco

Expanded release notes
Fix more missing HOME references

Signed-off-by: Andrew Page <admwiggin@gmail.com>
Merge pull request #8323 from crosbymichael/bump_v1.3.0

Bump to version 1.3.0
Merge branch 'release' into merge_release_v1.3.0
Change version to 1.3.0-dev

Signed-off-by: Michael Crosby <crosbymichael@gmail.com>
fix minor docs error

Signed-off-by: Tobias Gesellchen <tobias@gesellix.de>
Setting iptables=false should propagate to ip-masq=false

Signed-off-by: Jessica Frazelle <jess@docker.com>
Merge pull request #8609 from gesellix/patch-2

fix minor docs error
Merge pull request #8608 from crosbymichael/merge_release_v1.3.0

Merge release v1.3.0