remove legacy kernel version maintenance

[phillxnet]

Further to our transitional grubby dependency removal and the end of CentOS based rpm releases (3.9.2-57) we can now remove the remaining grubby calls/references system wide, most notably in our initrock mechanism: our overarching system initialisation routine. See set_def_kernel(). The associated 'supported kernel' checks are likewise now redundant as we no longer have a need to manage/maintain our kernel versions nor the low level tool (grubby) to enforce a kernel version change. This mechanism is a legacy artefact of our earlier days where we were required to update our base OS's kernel in order to achieve basic btrfs competency. This is no longer the case.

Summary of proposal:

• We report only the kernel version and remove all mechanisms of version evaluation or enforcement. N.B. all mechanisms proposed for removal are currently non functional/deactivated within our 'Built on openSUSE' base.

Previous issue/pr references for the grubby dependency removal:
"remove grubby dependency for kernel version reporting" #1955
"remove grubby dependency for kernel version reporting. Fixes #1955" #1956

Indicated log error for this issue:

Installing def-kernel.
def-kernel: Running 'grubby --set-default=/boot/vmlinuz-'4.12.4-1.el7.elrepo.x86_64''
/bin/sh: grubby: command not found

Code references:

src/rockstor/scripts/initrock.py

set_def_kernel()
delete_old_kernels()

buildout.cfg & testing.cfg

[def-kernel]

src/rockstor/system/osi.py

kernel_info()

src/rockstor/smart_manager/data_collector.py

send_kernel_info()

[phillxnet]

I am currently working on this issue.