support comments in pxe mapping file csv

**Is your feature request related to a problem? Please describe.**

It would be great if I could use comments in my pxe mapping file csv. Something like:
```
FUNCTIONAL_GROUP_NAME,GROUP_NAME,SERVICE_TAG,PARENT_SERVICE_TAG,HOSTNAME,ADMIN_MAC,ADMIN_IP,BMC_MAC,BMC_IP,IB_NIC_NAME,IB_IP
slurm_control_node_x86_64,grp0,SVCTAG01,,head01,a1:b2:c3:d4:e5:f6,172.16.107.52,a2:b3:c4:d5:e6:f7,172.17.107.52,,
slurm_node_x86_64,grp1,SVCTAG02,,compute01,b1:c2:d3:e4:f5:a6,172.16.107.43,b2:c3:d4:e5:f6:a7,172.17.107.43,,
login_node_x86_64,grp2,SVCTAG03,,login01,c1:d2:e3:f4:a5:b6,172.16.107.44,c2:d3:e4:f5:a6:b7,172.17.107.44,,
# Removing this node for maintaince
#login_compiler_node_x86_64,grp3,SVCTAG04,,login-compiler01,d1:e2:f3:a4:b5:c6,172.16.107.45,d2:e3:f4:a5:b6:c7,172.17.107.45,,
```

**Describe the solution you'd like**
A way to add comments to pxe mapping file.

**Describe alternatives you've considered**
Starting a comment line with a #: this is common practice in configuration files.

**Additional context**

Discussion on adding comments to csv: https://stackoverflow.com/questions/1961006/can-a-csv-file-have-a-comment

Start by locating the Python entry point that reads the PXE mapping CSV and the tests covering that parser. Check how a line beginning with # is currently handled, then add coverage showing such lines are ignored while valid mappings still load; done means the example file parses without treating the comment as a mapping.

Written by the indexing model from the issue text.