Shell execute command refactoring

* Refactored `execute` and `execute_save_output` commands;
* Cleaned-up `ProcessExecutionError` exception;
* Added unit tests.

Change-Id: I21ae3d6347fb56c057e15c863056e2f39c599743
cryptography is a arch_dependent package

Change-Id: I8320cb993c5b70a4de6de25e8f8273c156887d2b
Install openssl-devel at bootstrap

For some strange reason, openssl headers are required by some
dependencies (notably cryptography) on prepare, so they
should be installed in advance.

Change-Id: Id4dce0968f0c0c979f63ebe1b48f898f44c29ed1
Install doc files if they found for OpenStack clients

Instead of creating empty documentation files for client that do not
have them we now package them only if they exists. We also package
man page if it is installed to buildroot.

Closes-bug: #1284065
Change-Id: Ibe3bfd9793d1dfcdeb5c59d96725a9896595ad98
Merge "cryptography is a arch_dependent package"
Merge "Install doc files if they found for OpenStack clients"
Merge "Install openssl-devel at bootstrap"
Merge "Shell execute command refactoring"