Support comments in reserved peers file (#6004)

[axelchalon]

Closes #6004

I made the parser ignore lines starting with # as suggested by Rob, and added a test for this.

You can test this by running parity with --reserved-peers ./peers.txt and have ./peers.txt be, for example:

# This is the validating node, owned by XXX corp
enode://6f8a80d14311c39f35f516fa664deaaaa13e85b2f7493f37f6144d86991ec012937307647bd3b9a82abe2974e1407241d54947bbb39763a4cac9f77166ad92a0@172.0.0.1:30303
# This is another validating node, owned by YYY corp
enode://6f8a80d14311c39f35f516fa664deaaaa13e85b2f7493f37f6144d86991ec012937307647bd3b9a82abe2974e1407241d54947bbb39763a4cac9f77166ad92a1@172.0.0.1:30303
# This is a non-validating node, owned by ZZZZ corp
enode://6f8a80d14311c39f35f516fa664deaaaa13e85b2f7493f37f6144d86991ec012937307647bd3b9a82abe2974e1407241d54947bbb39763a4cac9f77166ad92a2@172.0.0.1:30303

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