add a boot-target type

[edaniszewski]

The boot_target endpoint previously checked that the device type was system. This adds a boot_target type, which is used by the IPMI plugin and will be used by the AMT plugin.

It changes up the validation function to allow multiple types for an endpoint. Updates tests.

related to #153

[codecov-io]

Codecov Report

Merging #154 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   89.77%   89.82%   +0.05%     
==========================================
  Files          41       41              
  Lines        1105     1111       +6     
==========================================
+ Hits          992      998       +6     
  Misses        113      113

Impacted Files	Coverage Δ
synse/routes/aliases.py	98.85% <100%> (ø)	⬆️
synse/const.py	100% <100%> (ø)	⬆️
synse/validate.py	100% <100%> (ø)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f14822d...5513a46. Read the comment docs.