Remove hardcoded "rhpkg".

[junaruga]

This fixes https://github.com/sclorg/rpm-list-builder/issues/66 and https://github.com/sclorg/rpm-list-builder/issues/83 .

• Added --pkg-cmd PKG_CMD command option PKG_CMD works in build type: mock, copr.
• Added download type: fedpkg

Specification is here https://github.com/sclorg/rpm-list-builder/issues/83#issue-245364831 .

[junaruga]

I tested below cases too as acceptance test just in case.

$ rpmlb \
  --download fedpkg \
  --build mock \
  --mock-config fedora-rawhide-x86_64 \
  --branch master \
  --verbose \
  tests/fixtures/recipes/joke.yml \
  joke 2>&1 | tee rpmlb.log

$ rpmlb \
  --download fedpkg \
  --build mock \
  --pkg-cmd fedpkg \
  --mock-config fedora-rawhide-x86_64 \
  --branch master \
  --verbose \
  tests/fixtures/recipes/joke.yml \
  joke 2>&1 | tee rpmlb.log

$ rpmlb \
  --download rhpkg \
  --build mock \
  --pkg-cmd rhpkg \
  --mock-config rhscl-2.4-rh-ror50-rhel-7-x86-64 \
  --branch rhscl-2.4-rh-ror50-rhel-7 \
  --verbose \
  tests/fixtures/recipes/ror.yml \
  rh-ror50 2>&1 | tee rpmlb.log

$ rpmlb \
  --download fedpkg \
  --build copr \
  --copr-repo rh-ror50-test \
  --branch master \
  --verbose \
  tests/fixtures/recipes/joke.yml \
  joke 2>&1 | tee rpmlb.log

$ rpmlb \
  --download rhpkg \
  --build mock \
  --mock-config rhscl-2.4-rh-ror50-rhel-7-x86-64 \
  --branch rhscl-2.4-rh-ror50-rhel-7 \
  --verbose \
  tests/fixtures/recipes/ror.yml \
  rh-ror50 2>&1 | tee rpmlb.log

[junaruga]

@khardix thanks for the review. I fixed those, and rebased on latest master branch.

[junaruga]

Merged with squash. The document is later.