Migrate rootwrap to privsep

As a part of the community goal [1], we should replace rootwrap in
favor of privsep.

Although the latest codes don't have a dependency on rootwrap for now,
but it might be happened to introduce a task with root privilege after.
In addition, there are some complex mechanism embeded in Tacker such as
in setuptools or configs under `etc/`. It's hard to drop and restore
them again. So, keep the mechanism active.

In this update, two methods used for tests are implemented in
`tacker/privileged/linux_cmd.py`, but can be used for general purpose.
For the test, it's also including rootwrap for backward compatibility
which will be removed in a future update. It also updates required
libs as bellow for oslo.privsep 2.4.0.
  - eventlet>=0.30.1
  - msgpack>=0.6.0
  - oslo.service>=2.5.0

[1] https://governance.openstack.org/tc/goals/selected/migrate-to-privsep.html

Partially-Implements: bp privsep-migration
Signed-off-by: Yasufumi Ogawa <yasufum.o@gmail.com>
Change-Id: Id8de4c2bae91718d6ba45ed523edc103f0b21718

lower-constraints.txt

@@ -24,7 +24,7 @@ docutils==0.14
 dogpile.cache==0.6.5
 dulwich==0.19.0
 enum-compat==0.0.2
-eventlet==0.18.2
+eventlet==0.30.1
 extras==1.0.0
 fasteners==0.14.1
 fixtures==3.0.0
@@ -52,7 +52,7 @@ Mako==1.0.7
 MarkupSafe==1.1
 monotonic==1.4
 mox3==0.25.0
-msgpack==0.5.6
+msgpack==0.6.0
 munch==2.2.0
 netaddr==0.7.18
 netifaces==0.10.6
@@ -72,10 +72,11 @@ oslo.log==3.36.0
 oslo.messaging==9.3.0
 oslo.middleware==3.31.0
 oslo.policy==3.6.0
+oslo.privsep==2.4.0
 oslo.reports==1.18.0
 oslo.rootwrap==5.8.0
 oslo.serialization==2.18.0
-oslo.service==1.24.0
+oslo.service==2.5.0
 oslo.upgradecheck==1.3.0
 oslo.utils==4.8.0
 oslo.versionedobjects==1.33.3