Search for heavy resonances decaying into a $\mathrm{W}$ or $\mathrm{Z}$ boson and a Higgs boson in final states with leptons and $\mathrm{b}$-jets in 36 $\mathrm{fb^{-1}}$ of $\mathrm{\sqrt s = 13}$ TeV $\mathrm{pp}$ collisions with the ATLAS detector