Pre check playbook should conform requirements from TEN-257

[mbukatov]

There is a initial version of prechecks.yml playbook which automates set of checks, expected to be run before installing Tendrl. The requirements are described in TEN-257.

This issue is tracking full compliance wrt TEN-257:

• Checks for duplicate machine IDs in storage nodes and fails with error if found (027c754)
• Checks for lack of NTP enablement and enables if missing (18e5693)
• Checks for lack of proper domain name setup and warns (b017203)
• Checks for lack of DNS setup and warns (b017203)
• Checks for adequate base OS version (Centos/RHEL 7.3 or higher) and fails with warning if missing
• Checks for adequate memory and disk (for Tendrl node only) and proceeds with warning if inadequate:
  • based on information from Tendrl-release-v1.5.0-(install-doc).md file
  • this check will require updates when the hw requirements are changed or specified in more details
  • the check is concerned with tendrl-server only as no details are directly given for other nodes, moreover given the current scope when preexisting Gluster Storage cluster is just imported, hw check of gluster storage machines is out of scope anyway

I decided against implementing (requirements listed below won't be added into pre check playbook):

• Checks for lack of proper host name and /etc/hosts setup and warns (must have a primary IP & host name in /etc/hosts)
• setup of ntp based time synchronization (when ntp sync is detected to be unconfigured)

Other changes compared to original requirements:

• The playbook run will just immediately fail when a problem is detected (via assert module), there are no checks that would give a warning only. The reason is that every problem needs a manual intervention and ansible is not suited for reporting warnings.
• This playbook is not going to change anything on the machines, there should be no side effects.

[mbukatov]

Checks for dns and hostname configuration will be implemented while I work on #30

[mbukatov]

Since commit f4a44ae, precheck playbook is part of site example playbook.