Add option to override MGS host

[JohnGarbutt]

Added the config:
DAC_MGS_HOST

By default it is localhost, which is existing behaviour where
MGS always lives on the primary brick host.

We found operational issues of when to clean out the MGS after
creating lots of filesystems. Eventually the MGS will fill up.
As a work around, we allow operators to specify a custom location
(host and device) for the MGS. This means you could easily rotate
a single MGS (via change of config followed by reboot of dacd)
that ensures the correct thing happens.

[codecov-commenter]

Codecov Report

Merging #132 into master will increase coverage by 0.15%.
The diff coverage is 76.47%.

@@            Coverage Diff             @@
##           master     #132      +/-   ##
==========================================
+ Coverage   50.44%   50.59%   +0.15%     
==========================================
  Files          31       31              
  Lines        1911     1925      +14     
==========================================
+ Hits          964      974      +10     
- Misses        875      879       +4     
  Partials       72       72              

Impacted Files	Coverage Δ
internal/pkg/config/filesystem.go	0.00% <0.00%> (ø)
internal/pkg/filesystem_impl/mount.go	42.22% <66.66%> (+0.55%)	⬆️
internal/pkg/filesystem_impl/ansible.go	45.29% <84.61%> (+2.16%)	⬆️

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